Method for continuous production of polyamide

ABSTRACT

Provided is a continuous production method of a polyamide with stabilized polymerization degree and good quality, particularly an aromatic-containing polyamide. A continuous production method of a polyamide, comprising (a) a raw material preparation step of individually melting a diamine and a dicarboxylic acid, or producing a salt of amine and carboxylic acid in water, (b) a raw material introduction step of continuously introducing the prepared raw materials into a tubular reaction apparatus, (c) an amidation step of passing the introduced raw materials through the tubular reaction apparatus, thereby effecting amidation to obtain a reaction mixture containing an amidated product and a condensed water, (d) an initial polymerization step of introducing the reaction mixture into a continuous reaction apparatus capable of separation and removal of water, and elevating the polymerization degree while separating and removing water at a temperature higher than the melting point of the finally obtained polyamide to obtain a polyamide prepolymer, and (e) a final polymerization step of introducing the polyamide prepolymer into a continuous reaction apparatus capable of separation and removal of water, and further elevating the polymerization degree at a temperature higher than the melting point of the finally obtained polyamide to obtain a polyamide adjusted to a desired relative viscosity [RV].

TECHNICAL FIELD

The present invention relates to a continuous production method of apolyamide with good quality and stable polymerization degree. Thecontinuous production method of the present invention can be applied toeither an aliphatic polyamide or an aromatic-containing polyamide but ispreferably applied to an aromatic-containing polyamide of whichproduction is in more difficult conditions.

Polyamides having an aromatic ring are excellent in the mechanicalstrength and dimensional stability and can be preferably used for film,sheet, packaging bag, bottle, engineering plastic, fiber and the like.

BACKGROUND ART

The polyamide resin is being widely used for usage such as film, sheet,packaging bag, engineering plastic and fiber, because of its excellentphysical and mechanical properties.

In such usage, aliphatic polyamides such as nylon 6 and nylon 66 havebeen heretofore predominantly used. However, the aliphatic polyamide ingeneral has drawbacks that a large dimensional change occurs between thewater absorption moisture absorption time and the drying time and sincethis is aliphatic, the elastic modulus is small and the softness isexcessively high. Therefore, a polyamide resin having higher performanceis being demanded. Under these circumstances, an aromatic dicarboxylicacid such as TPA (terephthalic acid) and IPA (isophthalic acid) iscopolymerized with a conventional aliphatic polyamide so as to attainhigh performance of the polyamide resin. For example, JP 59-155426 A andJP 62-156130 A disclose a polyamide resin in which TPA or IPA iscopolymerized.

However, the introduction of an aromatic ring into the polyamideskeleton generally leads to elevation of melting point or meltviscosity, and the production of polyamide is confined to severerconditions such as temperature condition and encounters moreacceleration in the production of gelled product, thermal degradation ordeterioration due to thermal decomposition reaction. The gelled productmay deposit in the polymerization rector to require more frequentcleaning operations or mingle into the resin, or the thermal degradationmay bring about reduction in the physical properties, as a result, ahigh-quality polyamide resin cannot be obtained.

These are mainly attributable to the long-term residence of polyamideresin in a high-temperature condition, and various production methodshave been proposed with an attempt to solve these problems. For example,according to the production methods disclosed in JP 60-206828 A, JP2-187427 A and JP 8-170895 A, an initial condensate is once taken out asa prepolymer so as to avoid the long-term residence under hightemperature, and this polymer is solid phase-polymerized at atemperature lower than the melting point of polymer, whereby the thermaldecomposition degradation is inhibited. However, these methods all are abatch-system production method and are undesirable in view of productionefficiency and also disadvantageous in that the quality readily differsamong batches.

As for the polyamide, a large number of aromatic-containing polyamidesare also known, where, for example, reduction of water absorption andelevation of elastic modulus are realized by using an aromatic diaminesuch as p-xylylenediamine (PXD) or m-xylylenediamine (MXD) as a rawmaterial.

The raw materials used in the production of a polyamide are generally adiamine and a dicarboxylic acid as in the production of 6,6-nylon. Inthis case, for elevating the polymerization degree to a level ofallowing for use as a product, it is important to control the molbalance between the diamine component and the dicarboxylic acidcomponent. This problem is generally overcome by employing a method ofcharging two components of diamine component and dicarboxylic acidcomponent in the form of an aqueous solution, and adjusting the pH toform a salt of amine and carboxylic acid. However, the salt formingmethod is disadvantageous in that since a large amount of water contentmust be removed so as to allow the polymerization reaction to proceed, alarge quantity of heat is necessary as compared with the amount ofproduction and moreover, the apparatus becomes large. Furthermore, whencontinuous production is intended, the pH adjustment performed everyeach batch takes time and therefore, the efficiency is low.

In order to solve these problems in the polymerization method using anaqueous solution of salt, a method of performing continuouspolymerization of polyamide without using water as a solvent has beenproposed.

For example, JP 10-509760 T employs a method of supplying a dicarboxylicacid excess component in the melted state to a multistage reactor, andadding the lacking diamine component to the reactor. However, in thismethod, the addition of diamine and the polymerization reaction must beperformed in parallel in the polymerization reactor and therefore, theapparatus structure becomes special and complicated.

JP 2001-200052 A discloses a continuous production method of apolyamide, which comprises a step of continuously supplying a slurrycomprising a xylylenediamine-containing diamine and a dicarboxylic acidto a ventless twin-screw extruder and heating it to allow for proceedingof amidation reaction, and a step of elevating the polymerization degreeof polyamide in a single-screw extruder with a vent while separating andremoving the condensed water produced by the amidation reaction.According to this patent publication, a slurry solution of diamine anddicarboxylic acid is prepared by a batch system at a low temperature of80° C. or less and then the polymerization reaction is initiated. Inthis method, the problem of large apparatus in the aqueous solutionpolymerization is overcome, but for preparing a slurry solution withoutcausing an amidation reaction rich in reactivity, the temperature andmoisture percentage must be strictly controlled and moreover, thepreparation of a homogeneous slurry solution takes time, giving rise toa problem in the productivity. The molecular weight of the polyamideobtained in Examples of this patent publication is low and approximatelyfrom 3,000 to 5,000.

JP 2002-516366 T discloses a continuous production method of nylon 66.According to this patent publication, a fused dicarboxylic acid and afused diamine are mixed in equimolar amounts by using a raw materialweighing system to produce a fused reaction mixture, the reactionmixture is passed to a non-ventilative reaction apparatus (staticin-line mixer) to form a first product flow containing polyamide andcondensed water, the first product flow is supplied to a ventilatedtank-type reaction vessel to remove the condensed water and form asecond product flow containing polyamide, the second product flow ismeasured by near infrared spectroscopy to determine relative amounts ofamine end group and carboxylic acid end group and based on the valuesobtained, the dicarboxylic acid weighing system and/or diamine weighingsystem are controlled.

JP 2002-516365 T discloses a control system for controlling the molbalance between fused dicarboxylic acid and fused diamine, where thebalance between carboxylic acid end group and amine end group in thepolymerization mixture is detected by using a near infrared spectrometerand based on the detection results, the mass flow rate of at least oneof the fused dicarboxylic acid and the fused diamine is adjusted.

However, when the mol balance between dicarboxylic acid and diamine iscontrolled by the feed back from the downstream polymerization mixtureto the upstream raw material supply part, time lag is caused and this isnot preferred in respect that the mol balance is difficult to exactlycontrol at all times. Also, such a control system is disadvantageouslycomplicated and costs high.

In order to express properties suitable for respective uses describedabove, a polyamide having a desired polymerization degree is necessary.Generally, in producing a polyamide, the polymerization degree ofproduct polyamide is determined by measuring its relative viscosity[RV]. The relative viscosity is one of the most important indices inconfirming the quality of polyamide.

The polymerization degree has a relationship with, for example, reactiontemperature, inner pressure (vacuum degree) of reactor, end groupconcentration of polymer (addition of end group adjusting agent such asacid anhydride), and moisture percentage in the gas phase at theinterface of fused polymer during reaction and therefore, a method ofadjusting the polymerization degree by changing any one of theseconditions is generally employed.

However, when the polymerization degree is intended to adjust by usingonly one of those conditions, various status changes are generated otherthan the polymerization degree and this may adversely affect the qualityof polyamide produced. For example, when only the degree of vacuum isadjusted to a high vacuum degree with an attempt to obtain a highpolymerization degree, the residence amount of polymer in thepolymerization reactor is varied due to change in the holdup broughtabout by bubbling and generation of contamination, change in theresidence time or the like is caused, as a result, a polyamide having anintended quality can be hardly obtained.

Also, when only the end group concentration of polymer is adjusted bythe addition of an end group adjusting agent with an attempt to obtain adesired polymerization degree, the end group adjusting agentdisadvantageously remains in the product polyamide in a larger amountalong with increase in the amount added of the end group adjustingagent.

DISCLOSURE OF THE INVENTION Object of the Invention

On the other hand, in the continuous production of polyamide, a polymerhaving a constant quality must be produced at all times by controllingthe real-time quality of polymer obtained and feeding it back to theproduction step. For this purpose, it is considered to on-line measurethe melt viscosity of polymer at the reactor outlet in the finalpolymerization step, decide the polymerization degree, and feedback theresults to the conditions in the polymerization step, therebycontrolling the polymerization degree of continuously producedpolyamide.

An object of the present invention is to provide a continuous productionmethod of a polyamide with stabilized polymerization degree.

An object of the present invention is to provide a continuous productionmethod of a polyamide with good quality, particularly anaromatic-containing polyamide. In particular, an object of the presentinvention is to provide a continuous production method of a polyamidewith excellent strength, good color tone and small water absorptionpercentage, which is suitable for film, sheet, packaging bag, bottle orthe like in uses such as food, beverage, medicament and cosmetic.

An object of the present invention is to provide a continuous productionmethod of a polyamide with good quality, for example, homogeneousaliphatic polyamide, particularly homogeneous aromatic-containingpolyamide. In particular, an object of the present invention is toprovide a continuous production method of a polyamide comprising anm-xylylenediamine as a diamine component, which is ensured withexcellent oxygen barrier property, good color tone and small waterabsorption percentage and suitable for film, sheet, packaging bag,bottle or the like in uses such as food, beverage, medicament andcosmetic.

SUMMARY OF THE INVENTION

The present inventors have found that when a self-cleaning horizontaltwin-screw reaction apparatus is used as the final polymerizationreaction apparatus and the operation of controlling the melt viscosityof polymer is performed in the final polymerization reaction apparatus,a polyamide having stabilized polymerization degree can be continuouslyobtained.

The present inventors have found that when a tubular reaction apparatusis used in the amidation reaction, the thermal degradation can besuppressed and a polyamide with good quality can be continuouslyobtained.

The present inventors have found that when a melted diamine and a melteddicarboxylic acid as raw materials are introduced into the reactionapparatus by using raw material supply means capable of automaticallycontrolling the mass flow rates of these two raw materials, an optimalmol balance can be achieved and a polyamide with good quality can becontinuously obtained.

The present invention includes the following inventions.

(1) A continuous production method of a polyamide, comprisingcontinuously producing a polyamide by melt polymerization using amultistage polymerization reaction apparatus,

wherein a self-cleaning horizontal twin-screw reaction apparatus is usedas a final polymerization reaction apparatus constituting the multistagepolymerization reaction apparatus,

wherein the final polymerization is effected while performing anoperation of purging inert gas inside the final polymerization reactionapparatus or while performing two or three operations selected from thegroup consisting of an operation of purging inert gas inside the finalpolymerization reaction apparatus, an operation of vacuating the finalpolymerization reaction apparatus, and an operation of adding an endgroup adjusting agent into the final polymerization reaction apparatus,and

wherein the melt viscosity of the polymer is controlled by continuouslymeasuring the melt viscosity of a polymer at an outlet of the finalpolymerization reaction apparatus by a viscometer and automaticallycontrolling at least one operation amount out of the inert gas purgedamount, the vacuum degree and the amount added of the end groupadjusting agent corresponding to the above-described operations so thatthe measured viscosity value becomes a value within a previously setdefinite range.

(2) The continuous production method of a polyamide as described in (1),wherein in performing two operations selected from the group consistingof the inert gas purging operation, the vacuum operation and theaddition operation of an end group adjusting agent, one operation amountout of two operation amounts is set as a fixed value and the otheroperation amount is automatically controlled.

(3) The continuous production method of a polyamide as described in (1),wherein in performing all the three operations selected from the groupconsisting of the inert gas purging operation, the vacuum operation andthe addition operation of an end group adjusting agent, two operationamounts out of three operation amounts are each set as a fixed value andonly the remaining one operation amount is automatically controlled, oronly one operation amount out of three operation amounts is set as afixed value and the other two operation amounts are automaticallycontrolled.

(4) The continuous production method of a polyamide as described in anyone of (1) to (3), wherein the inert gas has a moisture percentage of0.05 wt % or less.

(5) The continuous production method of a polyamide as described in anyone of (1) to (4), wherein the polyamide comprises an m-xylylenediamine(MXD) as a diamine component, and the m-xylylenediamine (MXD) content isat least 70 mol % based on the diamine component.

(6) The continuous production method of a polyamide as described in anyone (1) to (5), wherein a polyamide having a relative viscosity [RV] of1.6 to 4.0 is obtained.

The present invention further includes the following inventions.

(7) A continuous production method of a polyamide mainly comprising adiamine component unit and a dicarboxylic acid component unit, themethod comprising:

(a) a raw material preparation step of individually melting a diamineand a dicarboxylic acid or forming a salt of amine and carboxylic acidin water,

(b) a raw material introduction step of continuously introducing theprepared raw material into a tubular reaction apparatus,

(c) an amidation step of passing the introduced raw material through thetubular reaction apparatus, thereby effecting amidation to obtain areaction mixture containing an amidated product and a condensed water,

(d) an initial polymerization step of introducing the reaction mixtureinto a continuous reaction apparatus capable of separation and removalof water, and elevating the polymerization degree while separating andremoving water at a temperature higher than the melting point of thefinally obtained polyamide to obtain a polyamide prepolymer, and

(e) a final polymerization step of introducing the polyamide prepolymerinto a continuous reaction apparatus capable of separation and removalof water, and further elevating the polymerization degree at atemperature higher than the melting point of the finally obtainedpolyamide to obtain a polyamide adjusted to a desired relative viscosity[RV].

(8) The continuous production method of a polyamide as described in (7),wherein the tubular reaction apparatus used for the amidation step (c)has L/D of 50 or more, wherein the inner diameter of the tube is D (mm)and the length of the tube is L (mm).

(9) The continuous production method of a polyamide as described in (7)or (8), wherein the average residence time in the amidation step (c) isfrom 10 to 120 minutes.

(10) The continuous production method of a polyamide as described in anyone of (7) to (9), wherein the shear rate (γ) in the amidation step (c)is 0.1 (1/sec) or more and the shear stress (τ) is 1.5×10⁻⁵ Pa or more.

(11) The continuous production method of a polyamide as described in anyone of (7) to (10), wherein in the amidation step (c), the relativeviscosity [RV] of the reaction mixture is elevated by 0.05 to 0.6.

(12) The continuous production method of a polyamide as described in anyone of (7) to (11), wherein the average residence time in the initialpolymerization step (d) is from 10 to 150 minutes.

(13) The continuous production method of a polyamide as described in anyone of (7) to (12), wherein the continuous reaction apparatus in thefinal polymerization step (e) is a horizontal reaction apparatus.

(14) The continuous production method of a polyamide as described in anyone of (7) to (13), wherein the continuous reaction apparatus in thefinal polymerization step (e) is a self-cleaning horizontal twin-screwreaction apparatus.

(15) The continuous production method of a polyamide as described in anyone of (7) to (14), wherein the average residence time in the finalpolymerization step (e) is from 1 to 30 minutes.

(16) The continuous production method of a polyamide as described in anyone of (7) to (15), wherein the relative viscosity [RV] of the polyamideobtained in the final polymerization step (e) is from 1.6 to 4.0.

(17) The continuous production method of a polyamide as described in anyone of (7) to (16), wherein in the final polymerization step (e), therelative viscosity [RV] of the polyamide is controlled by an operationof purging inert gas inside the reaction apparatus, an operation ofadjusting vacuum degree in the reaction apparatus, an operation ofadding an end group adjusting agent into the reaction apparatus, or acombination thereof.

(18) The continuous production method of a polyamide as described in anyone of (7) to (17), wherein in the final polymerization step (e), thefinal polymerization is effected while performing an operation ofpurging inert gas inside the reaction apparatus or while performing twoor three operations selected from the group consisting of an operationof purging inert gas inside the reaction apparatus, an operation ofvacuating the reaction apparatus, and an operation of adding an endgroup adjusting agent into the reaction apparatus, and

wherein the melt viscosity of the polymer is controlled by continuouslymeasuring the melt viscosity of a polymer at an outlet of the finalpolymerization reaction apparatus by a viscometer and automaticallycontrolling at least one operation amount out of the inert gas purgedamount, the vacuum degree and the amount added of the end groupadjusting agent corresponding to the above-described operations so thatthe measured viscosity value becomes a value within a previously setdefinite range.

(19) The continuous production method of a polyamide as described in anyone of (7) to (18), wherein in the raw material preparation step (a),the atmospheric oxygen concentration at the preparation of raw materialis 10 ppm or less.

(20) The continuous production method of a polyamide as described in anyone of (7) to (19), wherein the polyamide comprises at least one memberselected from the group consisting of the following repeating units (I)to (V):

(21) The continuous production method of a polyamide as described in(20), wherein the polyamide comprises at least one member selected fromthe group consisting of the repeating units (I), (III) and (IV).

(22) The continuous production method of a polyamide as described in anyone of (7) to (21), wherein the polyamide comprises an m-xylylenediamine(MXD) as a diamine component, and the m-xylylenediamine (MXD) content isat least 70 mol % based on the diamine component.

The present invention further includes the following inventions.

(23) A continuous production method of a polyamide mainly comprising adiamine component unit and a dicarboxylic acid component unit, themethod comprising:

(a) a raw material preparation step of individually preparing a melteddiamine and a melted dicarboxylic acid,

(b) a raw material introduction step of continuously introducing themelted diamine and the melted dicarboxylic acid into a polymerizationreaction apparatus to get a diamine and a carboxylic acid together byusing raw material supply means comprising a raw material supply device,a mass flow rate measuring device provided on the downstream side of theraw material supply device and a control system of automaticallycontrolling the output of the supply device such that the mass flow ratemeasured by the mass flow rate measuring device becomes a previously setvalue, and

a polymerization step of polycondensing the diamine and the dicarboxylicacid introduced into the polymerization reaction apparatus.

(24) A continuous production method of a polyamide mainly comprising adiamine component unit and a dicarboxylic acid component unit, themethod comprising:

(a) a raw material preparation step of individually preparing a melteddiamine and a melted dicarboxylic acid,

(b) a raw material introduction step of continuously introducing themelted diamine and the melted dicarboxylic acid into a tubular reactionapparatus to get a diamine and a carboxylic acid together by using rawmaterial supply means comprising a raw material supply device, a massflow rate measuring device provided on the downstream side of the rawmaterial supply device and a control system of automatically controllingthe output of the supply device such that the mass flow rate measured bythe mass flow rate measuring device becomes a previously set value,

(c) an amidation step of passing the diamine and dicarboxylic acidgotten together through the tubular reaction apparatus, therebyeffecting amidation to obtain a reaction mixture containing an amidatedproduct and a condensed water,

(d) an initial polymerization step of introducing the reaction mixtureinto a continuous reaction apparatus capable of separation and removalof water, and elevating the polymerization degree while separating andremoving water at a temperature higher than the melting point of thefinally obtained polyamide to obtain a polyamide prepolymer, and

(e) a final polymerization step of introducing the polyamide prepolymerinto a self-cleaning horizontal twin-screw reaction apparatus capable ofseparation and removal of water, and further elevating thepolymerization degree at a temperature higher than the melting point ofthe finally obtained polyamide to obtain a polyamide adjusted to adesired relative viscosity [RV].

(25) The continuous production method of a polyamide as described in(24), wherein in the tubular reaction apparatus used for the amidationstep (c) has L/D or 50 or more, wherein the inner diameter of the tubeis D (mm) and the length of the tube is L (mm).

(26) The continuous production method of a polyamide as described in(24) or (25), wherein the average residence time in the finalpolymerization step (e) is from 1 to 30 minutes.

(27) The continuous production method of a polyamide as described in anyone (24) to (26), wherein the relative viscosity [RV] of the polyamideobtained in the final polymerization step (e) is from 1.6 to 4.0.

(28) The continuous production method of a polyamide as described in anyone of (24) to (27), wherein in the final polymerization step (e), therelative viscosity [RV] of the polyamide is controlled by an operationof purging inert gas inside the reaction apparatus, an operation ofadjusting vacuum degree in the reaction apparatus, an operation ofadding an end group adjusting agent into the reaction apparatus, or acombination thereof.

(29) The continuous production method of a polyamide as described in anyone of (24) to (28), wherein in the final polymerization step (e), thefinal polymerization is effected while performing an operation ofpurging inert gas inside the reaction apparatus or while performing twoor three operations selected from the group consisting of an operationof purging inert gas inside the reaction apparatus, an operation ofvacuating the reaction apparatus, and an operation of adding an endgroup adjusting agent into the reaction apparatus, and

wherein the melt viscosity of the polymer is controlled by continuouslymeasuring the melt viscosity of a polymer at an outlet of the finalpolymerization reaction apparatus by a viscometer and automaticallycontrolling at least one operation amount out of the inert gas purgedamount, the vacuum degree and the amount added of the end groupadjusting agent corresponding to the above-described operations so thatthe measured viscosity value becomes a value within a previously setdefinite range.

(30) The continuous production method of a polyamide as described in anyone of (24) to (29), wherein in the raw material preparation step (a),the atmospheric oxygen concentration at the preparation of raw materialis 10 ppm or less.

(31) The continuous production method of a polyamide as described in anyone (24) to (30), wherein the polyamide comprises an m-xylylenediamine(MXD) as a diamine component, and the m-xylylenediamine (MXD) content isat least 70 mol % based on the diamine component.

(32) The continuous production method of a polyamide as described in anyone (24) to (31), wherein the relative viscosity [RV] of the polyamideobtained in the final polymerization step (e) is from 1.6 to 4.0.

According to the present invention, a production method of a polyamidewith stabilized polymerization degree and good quality, and a continuousproduction method of a polyamide with stabilized polymerization degreeand good quality are provided.

According to the present invention, a continuous production method of apolyamide with good quality, particularly a homogeneousaromatic-containing polyamide, is provided. In the method of the presentinvention, a polyamide comprising an m-xylylenediamine as a diaminecomponent, which is ensured with excellent oxygen barrier property, goodcolor tone and small water absorption percentage and suitable for film,sheet, packaging bag, bottle or the like in uses such as food, beverage,medicament and cosmetic, is continuously and homogeneously produced.This polyamide comprising an m-xylylenediamine as a component can alsobe used for the reforming of a heteropolymer such as polyethyleneterephthalate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart roughly showing the process in the preferredcontinuous production method of a polyamide of the present invention.

FIG. 2 is a conceptual view of the system for automatically controllingthe operation amount in the present invention.

MODE FOR CARRYING OUT THE INVENTION

As for the production method of a polyamide by melt polymerization,production methods such as a batch system, a combination of batch systemand solid phase polymerization, and a continuous production using amultistage polymerization reactor are being employed. The melt viscositycontrol of the present invention may be applied to these conventionallyknown production methods but in particular, the melt viscosity controlof the present invention is preferably applied to the continuousproduction of a polyamide, because in the continuous production, it isnecessary to produce a polymer having a constant quality at all times bycontrolling the real-time quality of polymer obtained and feeding itback to the production step.

The continuous production method of the present invention can be appliedto either an aliphatic polyamide or an aromatic-containing polyamide,and such a polyamide comprises at least one member selected from therepeating units (I) to (V).

From the standpoint of reducing the water absorption percentage, it isimportant that the polyamide has an aromatic ring. The polyamidesuitable for this purpose comprises at least one member selected fromthe repeating units (I), (III) and (IV). The continuous productionmethod of the present invention can be preferably applied to anaromatic-containing polyamide of which production is in more difficultconditions.

Examples of the aromatic-containing polyamide include a polyamidecomprising 70 mol % or more of m-xylylenediamine (MXD) as an aminecomponent, which is a useful oxygen barrier material.

In view of oxygen barrier and water absorption, it is important that thepolyamide contains an m-xylylenediamine in an amount of 70 mol % or morebased on the diamine component. A smaller amount of m-xylylenediamine isadvantageous in the light of thermal degradation and color tone but fromthe standpoint of oxygen barrier, the m-xylylenediamine content must be70 mol % or more, preferably 75 mol % or more. On the other hand, fromthe aspect of water absorption, MXD itself has an aromatic ring and thewater absorption percentage is advantageously small as compared withaliphatic polyamides such as nylon 6 and nylon 66. However, the waterabsorption may be further improved by copolymerizing an aromaticdicarboxylic acid such as terephthalic acid or isophthalic acid.

Preferred examples of the polyamide include (1) a polyamide startingfrom TPA (terephthalic acid), HMDA (hexamethylenediamine) and CLM(caprolactam) and having the repeating units (I) and (V) (this polyamideis simply referred to as “HCT”), (2) a polyamide starting from IPA(isophthalic acid), ADA (adipic acid) and MXD (m-xylylenediamine) andhaving the repeating units (III) and (IV) (this polyamide is simplyreferred to as “MIA”), (3) a polyamide starting from TPA, IPA and HMDAand having the repeating unit (I) (this polyamide is simply referred toas “HIT”), (4) a polyamide starting from TPA, ADA and HMDA and havingthe repeating units (I) and (II) (this polyamide is simply referred toas “HTA”), (5) a polyamide starting from IPA, ADA and MXD and having therepeating units (III) and (IV) (this polyamide is simply referred to as“MIA”), (6) a polyamide starting from IPA, ADA, HMDA and MXD and havingthe repeating units (I), (II), (III) and (IV) (this polyamide is simplyreferred to as “HMIA”), (7) a polyamide starting from MXD and ADA andhaving the repeating unit (IV) (this polyamide is simply referred to as“MA”), and (8) a polyamide starting from MXD, ADA and TPA and having therepeating units (III) and (IV) (this polyamide is simply referred to as“MTA”).

As the amount of the aromatic dicarboxylic acid unit constituting thepolyamide increases, the polyamide is decreased in the water absorptionpercentage and increased in the dimensional stability and ahigh-elasticity and high-strength polyamide is obtained. In this way, alarge amount of aromatic dicarboxylic acid unit is advantageous in viewof physical and mechanical properties, but the introduction of aromaticring gives rise to elevation of melting point or melt viscosity of thepolyamide and the production conditions of polyamide become severer, asa result, the production of polyamide tends to be difficult.Furthermore, the operability at the shaping changes for the worse and aproduct with stabilized quality can be hardly obtained.

The amount of the aromatic dicarboxylic acid varies depending on thestructure of polyamide but in the case of a terephthalic acid, theamount thereof is preferably from 3 to 75 mol %, more preferably from 5to 70 mol %, based on the dicarboxylic acid component, and in the caseof an isophthalic acid, the amount thereof is preferably from 5 to 90mol %, more preferably from 10 to 85 mol %, based on the dicarboxylicacid. If the amount of terephthalic acid or isophthalic acid exceeds theupper limit in such a range, problems described above are caused,whereas if it is less than the lower limit, dimensional stability ormechanical property decreases.

In the case of the polyamide MA starting from m-xylylenediamine MXD andadipic acid ADA and having the repeating unit (IV), it is important inview of oxygen barrier and water absorption that the polyamide contains70 mol % or more of m-xylylenediamine based on the diamine component. Asmaller amount of m-xylylenediamine is advantageous in the light ofthermal degradation and color tone but from the standpoint of oxygenbarrier, the m-xylylenediamine content must be 70 mol % or more,preferably 75 mol % or more. On the other hand, from the aspect of waterabsorption, MXD itself has an aromatic ring and the water absorptionpercentage is advantageously small as compared with aliphatic polyamidessuch as nylon 6 and nylon 66. However, the water absorption may befurther improved by copolymerizing an aromatic dicarboxylic acid such asterephthalic acid or isophthalic acid.

In the production of a polyamide of the present invention, a rawmaterial having polyamide-forming ability, other than theabove-described diamine, dicarboxylic acid and lactam, may becopolymerized as needed in view of performance required of thepolyamide.

Examples of the diamine component include aliphatic diamines such asethylenediamine, 1-methylethyldiamine, 1,3-propylenediamine,tetramethylenediamine, penta-methylenediamine, heptamethylenediamine,octamethylene-diamine, nonamethylenediamine, decamethylenediamine,undecamethylenediamine and dodecamethylenediamine. Other examplesinclude cyclohexanediamine, bis(4,4′-aminohexyl)methane andp-xylylenediamine.

Examples of the dicarboxylic acid component include aliphaticdicarboxylic acids such as malonic acid, succinic acid, glutaric acid,sebacic acid, pimelic acid, suberic acid, azelaic acid, undecanoic acid,undecadionic acid, dodecadionic acid and dimer acid,1,4-cyclohexanedicarboxylic acid, p-xylylenedicarboxylic acid,m-xylylenedicarboxylic acid, phthalic acid, 2,6-naphthalenedicarboxylicacid and 4,4′-diphenyldicarboxylic acid.

Other than these diamine and dicarboxylic acid components, a lactam suchas laurolactam, aminocaproic acid and aminoundecanoic acid, and anaminocarboxylic acid may also be used as a copolymerization component.

In view of physical and mechanical properties of the shaped articleobtained as well as operation stability, the relative viscosity [RV] ofthe polyamide resin is preferably from 1.6 to 4.0. If the [RV] is lessthan 1.6, this not only results in giving a shaped article having poormechanical properties but also tends to greatly affect the operationalaspect, such as generation of vent-up, difficult removal of polymerstrand and cracking at the formation into chips, whereas if [RV] exceeds4.0, the melt viscosity elevates to render the shaping conditionsseverer, as a result, a shaped article having stabilized quality tendsto be difficultly obtained and the product cannot be expected to havephysical properties worthy of the labors required. Furthermore,elevation of the [RV] to exceed 4.0 requires increase of the purgedamount with inert gas or application of a high vacuum degree and thisdisadvantageously incurs cost rise or unstable operation such asvent-up. The [RV] is more preferably from 1.9 to 3.8.

The amino end group concentration [AEG] and carboxyl end groupconcentration [CEG] of the polyamide resin each correlates with theabove-described relative viscosity [RV] or a molar ratio betweendicarboxylic acid and diamine. The [AEG] and [CEG] of polyamide resingenerally used in practice are both 200 (meq/kg) or less. These valuesare appropriately selected in accordance with use end of the polyamide.

According to the continuous production method of the present invention,the [AEG] and [CEG] can be effectively prevented from dispersion. Thedipsersions of [AEG] and [CEG] are expressed by their standard deviationor fluctuation range (difference between maximum value and minimum valuein aging) and as these values re smaller, the quality of polyamide isadvantageously more homogeneous. The standard deviation of [AEG] ispreferably 10 (meq/kg) or less and the standard deviation of [CEG] isalso preferably 10 (meq/kg) or less.

The polyamide resin is less yellow-tinted and superior in the color toneas the color tone [Co-b] value is smaller. Generally, when the [co-b] isfrom −3 to 3, there arises no problem as a product. If the [Co-b] isless than −3, the difference provided in the obtained color tone is outof the visible range and unworthy of the labors required to achieve sucha color tone value and this is meaningless, whereas if the [Co-b]exceeds 3, the yellow tinting increases and this worsened color tone isclearly viewed still in the product produced. The [Co-b] is preferablyfrom -2.5 to 2.8.

The water absorption percentage of the polyamide resin is an index forthe dimensional change of a shaped article between drying time andmoisture absorption time. The water absorption percentage is preferablysmaller, because the dimensional change becomes larger as the waterabsorption percentage is higher. The water absorption percentage ispreferably 7% or less, more preferably 6.7% or less. Since thedimensional stability is more excellent as the water absorptionpercentage is lower, the lower limit of the water absorption percentageis not particularly specified. However, in view of properties inherentin polyamide, it is technically difficult to obtain a polyamide having awater absorption percentage of 3.5% or less.

The continuous production method of a polyamide of the present inventionis described below by referring to FIG. 1. FIG. 1 is a flow chargeroughly showing the preferred process in the continuous productionmethod of a polyamide of the present invention. In FIG. 1, thecontinuous production method of a polyamide comprises a raw materialpreparation step (a), a raw material introduction step (b), an amidationstep (c), an initial polymerization step (d) and a final polymerizationstep (e). That is, in this example, the polymerization process comprisesan amidation step (c), an initial polymerization step (d) and a finalpolymerization step (e).

Raw Material Preparation Step:

The raw material preparation step includes a method of individuallymelting a diamine and a dicarboxylic acid and supplying each meltedmonomer directly to the amidation step, and a method of forming a saltof amine and carboxylic acid in water and supplying an aqueous solutionof the salt to the amidation step.

1. Method of Directly Supplying Melted Monomer

The raw material preparation equipment mainly comprises a dicarboxylicacid melting tank (11), a storage tank (12) for the dicarboxylic acidmelted liquid, a diamine melting tank (13) and a storage tank (14) forthe diamine melted liquid. An example of this case is shown in FIG. 1.

The melting temperature and storing temperature of dicarboxylic acideach is suitably from its melting point to melting point +50° C. (atemperature 50° C. higher than the melting point). If the meltingtemperature and storing temperature are excessively high, thisdisadvantageously induces thermal decomposition or degradation of rawmaterials, whereas if these temperatures are too low, non-uniformmelting results to cause bad precision in the raw material supply to theamidation step and this is not preferred. The melting temperature andstoring temperature are preferably from melting point +5° C. to meltingpoint +25° C. The same applies to diamine and the melting temperatureand storing temperature of diamine each is suitably from its meltingpoint to melting point +50° C., preferably from melting point +5° C. tomelting point +25° C. Usually, a diamine which is liquid at ordinarytemperature is preferred.

For both of the dicarboxylic acid and the diamine, the melting tank andstorage tank at the raw material preparation are preferably laid in aninert gas atmosphere, for example, nitrogen gas atmosphere, so as toprevent thermal oxidation decomposition or thermal decomposition. Atthis time, from the standpoint of preventing mingling of outer air, theinert gas atmosphere is preferably under pressure of 0.05 to 0.8 MPa,more preferably from 0.1 to 0.6 MPa.

In this raw material preparation step, the dicarboxylic acid and thediamine are individually melted. These individually melted dicarboxylicacid and diamine are continuously supplied at a predetermined molarratio to the amidation step in the raw material introduction step.

2. Salt Forming Method

This is an embodiment not shown in FIG. 1. The salt forming method isadvantageous for the production of a polyamide starting from adicarboxylic acid not having a melting point, such as terephthalic acidand isophthalic acid. The raw material preparation equipment mainlycomprises a salt forming tank, a storage tank for an aqueous solution ofthe obtained salt, and a supply pump.

The salt forming tank is the equipment for uniformly mixing the rawmaterials of polyamide, such as dicarboxylic acid, diamine, lactam andaminocarboxylic acid, in water to obtain an aminocarboxylate solution.In this salt forming step, the molar ratio between amino group andcarboxyl group may be arbitrarily adjusted according to the desiredphysical properties of the product. However, if the molar ratio largelydeviates from amino group/carboxyl group =1 (by mol), not only apolyamide having a desired [RV] cannot be obtained but also troubles inview of equipment, such as occurrence of vent-up of polymer, aredisadvantageously caused.

At this raw material preparation, an alkali metal compound or aphosphorus compound may be added for the purpose of preventing thermaloxidation decomposition or as a polymerization catalyst.

The salt concentration of aminocarboxylate produced in the salt formingstep varies depending on the kind of polyamide and is not particularlylimited, but in general, the salt concentration is preferably from 30 to90 wt %. If the salt concentration exceeds 90 wt %, the saltprecipitates with slight fluctuation in the temperature to clog a pipeor the equipment must be resistant against high temperature and highpressure so as to elevate the solubility of salt and this isdisadvantageous in view of cost, whereas if the salt concentration isless than 30 wt %, the amount of water evaporated after the initialpolymerization step increases and this is not only disadvantageous inview of energy but also gives rise to cost rise due to reduction inproductivity. The salt concentration is preferably from 35 to 85 wt %.

The conditions in the salt forming step vary depending on the kind ofpolyamide and the salt concentration, but in general, the temperature isfrom 60 to 180° C. and the pressure is from 0 to 1 MPa. If thetemperature exceeds 180° C. or the pressure exceeds 1 MPa, the equipmentmust have resistance against high temperature and high pressure andcosts high and this is not preferred, whereas if the temperature is lessthan 60° C. or the pressure is less than 0 MPa, not only troubles suchas clogging of a pipe due to precipitation of salt are caused but alsothe salt concentration can be hardly elevated, resulting in reduction ofproductivity. The conditions are preferably such that the temperature isfrom 70 to 170° C. and the pressure is from 0.05 to 0.8 MPa, morepreferably such that the temperature is from 75 to 165° C. and thepressure is from 0.1 to 0.6 MPa.

The storage tank for the aqueous solution of salt is fundamentallysufficient if precipitation of salt does not occur, and the conditionsfor the salt forming step can be applied as-is.

The thus-prepared aqueous solution of salt is continuously supplied tothe amidation step by a supply pump in the raw material introductionstep. The supply pump used here must have excellent quantitativity. Thefluctuation in the supply amount works out to fluctuation of process inthe amidation step, as a result, the obtained polyamide has a largedeviation of relative viscosity [RV] and the quality thereof is notstabilized. In this meaning, the supply pump is preferably a plungerpump excellent in the quantitativity.

3. Oxygen Concentration at Raw Material Preparation

The atmospheric oxygen concentration at the raw material preparationgreatly affects the color tone of polyamide obtained. In particular,this tendency is outstanding for the polyamide using anm-xylylenediamine as a raw material. When the atmospheric oxygenconcentration at the raw material preparation is 10 ppm or less, therearises no problem, but if the oxygen concentration exceeds 10 ppm, theobtained polyamide is strongly yellow-tinted and the grade of producttends to decrease. The lower limit of the oxygen concentration is notparticularly specified but, for example, 0.05 ppm or more. In theproduction of polyamide, an oxygen concentration of less than 0.05 ppmraises no problem, but only an excessively complicated step of removingoxygen is necessary for achieving an oxygen concentration of less than0.05 ppm and other physical properties including color tone are scarcelyaffected. The oxygen concentration is preferably from 0.05 to 9 ppm,more preferably from 0.05 to 8 ppm.

In the present invention, a process of supplying raw materials to apreparation tank (melting tank or raw material salt forming tank) fromwhich oxygen is previously removed to an oxygen concentration of 10 ppmor less, a process of charging raw materials into a preparation tank(melting tank or raw material salt forming tank) and then removingoxygen inside the preparation tank to provide an atmosphere having anoxygen concentration of 10 ppm or less, or a combination thereof may beused. This may be selected in the light of equipment or operation. Theatmosphere inside the storage tank is also preferably made to have anoxygen concentration of 10 ppm or less.

The removal of oxygen may be performed by a vacuum displacement method,a compression displacement method or a combination thereof. As for thevacuum degree or compression degree applied to displacement and thenumber of displacements, conditions ensuring highest efficiency inachieving the desired oxygen concentration may be selected.

Raw Material Introduction Step:

In the raw material introduction step, in the case of the method ofdirectly supplying the melted monomers, the dicarboxylic acid anddiamine individually melted in the raw material preparation step arecontinuously introduced into an inlet (22) of a tubular reactionapparatus for the amidation step by using raw material supply meansthrough respective pipe lines from respective storage tanks (12) and(14), thereby getting a diamine and a dicarboxylic acid together.

In the case of the salt forming method, the salt aqueous solutionprepared in the raw material preparation step is continuously introducedinto the inlet (22) of a tubular reaction apparatus for the amidationstep by a supply pump through a pipe line.

The raw material supply means preferably comprises raw material supplydevices (15) and (16), mass flow rate measuring devices (17) and (18)provided on the downstream side of (preferably immediately after) theraw material supply devices (15) and (16), respectively, and a controlsystem (19) of automatically controlling the outputs of the supplydevices (15) and (16) such that the mass flow rate measured by each ofthe mass flow rate measuring devices (17) and (18) becomes a previouslyset value. The mass flow rate measuring devices (17) and (18) must beprovided on the upstream side than the confluence of diamine anddicarboxylic acid.

For performing high-precision supply by using the automatic controlsysem (19), it is important that the supply precision of the rawmaterial supply devices (15) and (16) themselves is within 1.5%. Theprecision of the raw material supply devices (15) and (16) is preferably1% or less, more preferably 0.5% or less, still more preferably 0.25% orless. The raw material supply device is preferably a plunger pumpbecause of its excellent quantitativity.

In the present invention, mass flow rate measuring devices (17) and (18)are provided on the downstream side of, preferably immediately after,the raw material supply devices (15) and (16), respectively. As for themass flow rate measuring devices (17) and (18), a flowmeter such asCoriolis type may be used.

The mass flow rates of respective materials continuously fed out fromthe raw material supply devices (15) and (16) are continuously measuredby the mass flow rate measuring devices (17) and (18). The measured massflow rate of each raw material is sent to the control unit (19). In thecontrol unit (19), respective mass flow rate values of dicarboxylic acidand diamine are previously set to give a desired mol balance. When themeasured mass flow rate value of each raw material deviates from thisset value, the control unit (19) sends a control signal to each rawmaterial supply device (15) or (16) for controlling the output of thedevice to return each mass flow rate to the set value, whereby theoutput of each raw material supply device (15) or (16) is controlled. Inthis way, the mass flow rate value of each raw material is automaticallycontrolled, and a dicarboxylic acid and a diamine are continuouslyintroduced into the inlet (22) of the tubular reaction apparatus whilekeeping the desired mol balance according to the end use of polymer atall times. Thus, in the raw material introduction step of the presentinvention, the mass flow rate of each raw material continuously fed outfrom the raw material supply device is continuously measured by the massflow rate measuring device, and each raw material is continuouslyintroduced at a predetermined molar ratio into the polymerizationreaction apparatus to get a diamine and a dicarboxylic acid together,while automatically controlling the output of the supply device suchthat the measured mass flow rate becomes a previously set value.

When the mass flow rate measuring devices (17) and (18) are providedimmediately after the raw material supply devices (15) and (16),respectively, the time lag between the supply device (15) or (16)and themass flow rate measuring device (17) or (18) is remarkably decreased andthis is very preferred because the measured mass flow rate value of eachmaterial is promptly reflected in the supply device (15) or (16).

Here, it is important to use a mass flow rate measuring device as theflow rate measuring device. If a volume flow rate measuring device isused, the control cannot be performed exactly, because the volume of rawmaterial is affected by the ambient temperature.

Amidation Step:

In the amidation step, the diamine and dicarboxylic acid continuouslyintroduced into the inlet (22) of a tubular reaction apparatus and gottogether, or a salt aqueous solution (in the case of the salt formingmethod) are(is) passed through a tubular reaction apparatus (21),thereby effecting amidation to obtain a reaction mixture containing anamidated product of low polymerization degree and a condensed water. Inthe tubular reaction apparatus (21), separation and removal of water arenot performed.

In the tubular reaction apparatus (21), assuming that the inner diameterof the tube is D (mm) and the length of the tube is L (mm), L/D ispreferably 50 or more. The tubular reaction apparatus is advantageous inthat, for example, the liquid level need not be controlled in view ofits structure, the plug flow property is high, the pressure resistanceis excellent, and the equipment cost is low. In the case where L/D isless than 50, if L is small, the residence time of the reaction mixtureflow is shortened and the degree of elevation in the relative viscosity[RV] is small, whereas if D is large, the plug flow property decreasesand a residence time distribution is produced, as a result, the desiredfunction is not fulfilled. The upper limit of L/D is not particularlylimited but is about 3,000 in the light of residence time and degree ofelevation in the relative viscosity [RV]. The lower limit of L/D is morepreferably 60 or more, still more preferably 80 or more, and the upperlimit thereof is more preferably 2,000 or less, still more preferably1,000 or less. The lower limit of L is preferably 3 m or more, morepreferably 5 m or more, and the upper limit thereof is preferably 50 mor less, more preferably 30 m or less.

The reaction conditions in the tubular reaction apparatus (21) varydepending on the structure of polyamide and the objective polymerizationdegree but, for example, the inner temperature is from 110 to 310° C.,the inner pressure is from 0 to 5 MPa, and the average residence time intube of the reaction mixture is from 10 to 120 minutes. Thepolymerization degree of the amidated product can be controlled by theinner temperature, inner pressure and average residence time.

If the average residence time is less than 10 minutes, thepolymerization degree of the amidated product with low polymerizationdegree decreases, as a result, entrainment, vent-up or the like occursin the later step and the operation tends to be unstable, whereas if theaverage residence time exceeds 120 minutes, the amidation reachesequilibrium to allow for no elevation of [RV] any more and in themeantime, the thermal degradation disadvantageously proceeds. Theaverage residence time is preferably from 12 to 110 minutes, morepreferably from 15 to 100 minutes. The average residence time can becontrolled by adjusting the tube inner diameter D or tube length L ofthe tubular reaction apparatus or changing the supply amount of rawmaterial.

The polycondensation reaction in the amidation step preferably causesthe relative viscosity [RV] of the reaction mixture to increase by 0.05to 0.6 between inlet (22) and outlet (23) of the tubular reactionapparatus (21). If the increase of [RV] is less than 0.05, similarly tothe case having a short residence time, the polymerization degree of theamidated product is low, as a result, entrainment, vent-up or the likeoccurs in the later step and the operation tends to be unstable, whereasif the increase of [RV] exceeds 0.6, thermal degradation readilyproceeds due to the effect of coexisting condensed water (in the case ofsalt forming method, water used for the formation of salt, and condensedwater). Furthermore, the reaction mixture excessively elevated in theviscosity gives rise to clogging of a pipe and this may adversely affectthe operation. The increase of [RV] in the amidation step is preferablyfrom 0.15 to 0.5, more preferably from 0.2 to 0.4.

In order to assure the plug flow property in the amidation step, it ispreferred that the shear rate (γ) is 0.1 (1/sec) or more and the shearstress (τ) is 1.5×10⁻⁵ Pa or more. If either one of the shear rate andshear stress decreases the above-described range, the residence timedistribution of the reaction mixture is broadened and coloration ofpolyamide or fluctuation of process may occur. The shear rate (γ) ispreferably 0.3 or more and the shear stress (τ) is preferably 2.0×10⁻⁵Pa or more. These are not particularly limited in the upper limit butusually, the shear rate (γ) is 100 (1/sec) or less and the shear stress(τ) is 3×10⁻² Pa or less.

Initial Polymerization Step:

In the initial polymerization step, the reaction mixture containing anamidated product with low polymerization degree and a condensed water(in the case of salt forming method, also water used for the formationof salt), obtained in the amidation step, is introduced into acontinuous reaction apparatus capable of separation and removal ofwater, and the polymerization degree thereof is elevated whileseparating and removing water at a temperature higher than the meltingpoint of the finally obtained polyamide to obtain a polyamideprepolymer.

In the initial polymerization step, the equipment used may be a verticalstirring tank, a centrifugal thin-film evaporator or the like but ispreferably a vertical stirring tank (31) in which the reactionconditions can be easily controlled. The vertical stirring tank (31) isconstructed to continuously receive the reaction mixture from the outlet(23) of amidation step, comprise a separation and removal device (32),and continuously discharge the polyamide prepolymer from the bottom(33).

As for the reaction conditions in the initial polymerization step, forexample, the inner temperature is from the melting point (Tm) of thefinally obtained polyamide to Tm+90° C., the inner pressure is from 0 to5 MPa, and the average residence time is from 10 to 150 minutes. Thereaction conditions are preferably such that the inner temperature isfrom the melting point (Tm) of polyamide to Tm+80° C., the innerpressure is from 0 to 4 MPa, and the average residence time is from 15to 140 minutes, and more preferably such that the inner temperature isfrom the melting point (Tm) of polyamide to Tm+70° C., the innerpressure is from 0 to 3.5 MPa, and the average residence time is from 20to 130 minutes. If the reaction conditions deviate from these ranges,the polymerization degree achieved may be excessively low or thermaldegradation or reduction of productivity may occur and this is notpreferred. The polymerization degree of the polyamide prepolymer can becontrolled by the inner temperature, inner pressure or average residencetime.

Final Polymerization Step:

In the final polymerization step, the polyamide prepolymer obtained inthe initial polymerization step is introduced into a continuouspolymerization reactor capable of separation and removal of water, andthe polymerization degree thereof is further elevated at a temperaturehigher than the melting point of the finally obtained polyamide toobtain a polyamide adjusted to a desired relative viscosity [RV].

The continuous reaction apparatus used in the final polymerization stepis preferably a horizontal twin-screw reaction apparatus havingself-cleaning property. In a horizontal twin-screw reaction apparatusgenerally used for the polymerization reaction (see, for example, JP50-21514 B and JP 48-84781 A), the residence amount in the kettle variesand therefore, when this apparatus is used over a long time for theproduction of a polyamide, in which thermal change readily occurs, thequality of the product polyamide is disadvantageously impaired due tostaining on the inner wall of kettle.

In order to reduce such adverse effects, a single-screw extruder ortwin-screw extruder having a property of self-cleaning the inner wall ofkettle can be used. A twin-screw extruder is generally preferred becauseof its good reaction efficiency and certain level of self-cleaningproperty. However, in the twin-screw extruder, not only the entireinside of the apparatus cannot be vacuumized but also in the case of amaterial with low melt viscosity, vent-up readily occurs. Furthermore,this apparatus is disadvantageous in that the temperature control isdifficult due to high shear forth and the latitude of residence time islimited. Moreover, in order to prolong the residence time, there ariseproblems such as large apparatus and high equipment cost.

Therefore, the continuous reaction apparatus used here is preferably aself-cleaning horizontal twin-screw reaction apparatus (41), forexample, SCR manufactured by Mitsubishi Heavy Ind., Ltd. In theself-cleaning horizontal twin-screw reaction apparatus (41), blades(rotors) twistedly superposed with a slight clearance are constitutingtwo parallel driving shafts, and these two parallel driving shafts arerotated in the same direction. The clearance between a blade and a bladeis preferably smaller because the cleaning effect of blades with eachother is higher. In the self-cleaning horizontal twin-screw reactionapparatus, the clearance between a blade and a blade greatly changesaccording to the size of apparatus but in the case of a reactionapparatus having an inner volume of about 0.15 m³, the clearance betweena blade and a blade is preferably 50 mm or less, more preferably 20 mmor less, still more preferably 10 mm or less.

In the self-cleaning horizontal twin-screw reaction apparatus, theclearance between the blade and the inner wall is small as compared withgeneral horizontal twin-screw reactors and therefore, the inner wall iscleaned along with the rotation of driving shafts. The clearance betweenthe blade and inner wall varies depending on the size of apparatus butin the case of a reaction apparatus having an inner volume of about 0.15m³, the clearance between the blade and the inner wall is preferably 15mm or less, more preferably 10 mm or less, still more preferably 5 mm orless.

In terms of the small clearance between the blade and the inner wall,SCR is the same as the twin-screw extruder, but as compared with thetwin-screw extruder having a fairly large clearance between a blade anda blade and rotating the driving shafts in opposite directions, only aslight clearance is present between a blade and a blade and the paralleltwo driving shafts rotate in the same direction, so that the effect ofcleaning the blades with each other can be more elevated. By virtue ofthe self-cleaning effect, reduction in the adherence of scales andenhancement of quality due to reduced contamination are obtained andtherefore, SCR is suitably used for the production of a polyamide, inwhich thermal degradation readily occurs during reaction. Unlike thetwin-screw extruder, the inside of the apparatus can be entirelyvacuumized and therefore, a vacuum can be applied also in the case of amaterial with low melt viscosity. Furthermore, SCR is advantageous inthat the heat generation due to shear force is small, the residence timeis relatively long, the ability to adapt to fluctuation of viscosity orflow rate is high, and the width of production possibility viscosity iswide. Also, in view of equipment, SCR is advantageous in thatcompactification is possible as compared with the twin-screw extrude,and the cost is low.

The reaction conditions in the final polymerization step vary dependingon the kind of polyamide and the desired relative viscosity [RV], butthe resin temperature is from the melting point (Tm) of polyamide toTm+80° C., preferably from the melting point to Tm+70° C. If the resintemperature exceeds Tm+80° C., deterioration of polyamide is readilyaccelerated and gives rise to reduction in physical properties orcoloration, whereas if it is less than Tm, there is a risk that thepolyamide is solidified and causes damage to the reaction apparatus. Theaverage residence time in the continuous reactor varies depending on thekind of polyamide, the desired relative viscosity [RV], the vacuumdegree, the addition of an acid anhydride compound described later, theinert gas purging described later, or the like but is preferably from 1to 30 minutes. If the average residence time is less than 1 minute, apolyamide having [RV] of 1.6 to 4.0 is difficult to obtain, whereas ifthe average residence time exceeds 30 minutes, the amount of the polymersupplied to the continuous reactor must be decreased and this causesserious reduction in the productivity. The average residence time ispreferably from 1.5 to 25 minutes, more preferably from 2 to 20 minutes.

The screw rotation number (rpm) of the reaction apparatus SCR lessaffects the polymerization reaction or average residence time unlike thetwin-screw extruder and may be appropriately selected, but in general, arotation number of 20 to 150 rpm is employed.

In the final polymerization step, the relative viscosity [RV] as anindex for the polymerization degree of polyamide is controlled. Therelative viscosity [RV] of the obtained polyamide and the melt viscosityof polymer at the reactor outlet in the final polymerization step arecorrelated and therefore, the relative viscosity [RV] can be controlledby controlling the melt viscosity of polymer.

In the present invention, it is preferred to continuously measure themelt viscosity of a polymer at the outlet (45) of the finalpolymerization reactor by a viscometer (50) and feedback the measuredresults to the polymerization step conditions so that the measuredviscosity value becomes a value within a previously set definite range.

The conditions in the polymerization step for controlling the meltviscosity of polymer include (1) the purged amount in an inactive gaspurging operation inside the reactor, (2) the vacuum density in a vacuumoperation in the reactor, and (3) the added amount in an additionoperation of an end group adjusting agent into the reactor. These aredescribed below.

The inert gas purging operation accelerates the polymerization reactionand at the same time, can control the melt viscosity by adjusting thepurged amount. The inert gas purging is performed from the inert gaspurging port (42). The purged amount with inert gas varies depending onthe desired melt viscosity and polymerization conditions such astemperature, but the purged amount is preferably 10 L or less, forexample, from 0.005 to 10 L, per 1 kg of the polymer. If the purgedamount exceeds 10 L/kg, the amount of inert gas used becomes excessivelylarge due to activity of accelerating the polymerization reaction andthis causes cost rise. The purged amount is preferably from 0.005 to 9.5L/kg, more preferably from 0.01 to 9 L/kg. In the case where the desiredRV can be obtained even without the inert gas purging, it is alsopossible not to perform the purging (that is, the purged amount is 10L/kg). The inert gas is not limited in the kind as long as it isinactive to the polyamide production reaction, but a nitrogen gas isadvantageous in view of safety and cost.

The moisture percentage of the inert gas is 0.05 wt % or less and thisis important. If the moisture percentage exceeds 0.05 wt %, theelevation of melt viscosity is retarded to adversely affect theproductivity and also, the gelling is caused. The moisture percentage ispreferably 0.03 wt % or less, more preferably 0.01 wt % or less. Theoxygen concentration in the reaction atmosphere is preferably 0.1 wt %or less, more preferably 0.001 wt % or less.

The polymerization reaction can be accelerated also by a vacuumoperation, and the reaction rate and melt viscosity can be controlled byadjusting the vacuum degree. This operation is performed through thevacuum port (43). The reaction of producing a polyamide is acondensation reaction between a carboxylic acid and an amine and byremoving water produced, the polymerization reaction is accelerated. Thevacuum degree applied in the final polymerization step varies dependingon the desired melt viscosity and polymerization conditions but is from150 to 1,200 hPa. If the vacuum degree is less than 150 hPa, vent-up ofpolymer or clogging of pipe may occur in the final polymerization stepand stable operability cannot be expected, whereas if it exceeds 1,200hPa, the vacuum degree is not so effective but rather the rate inachieving the desired melt viscosity becomes low to cause decrease inthe productivity and depending on the case, the desired melt viscositymay not be achieved. The vacuum degree is preferably from 200 to 1,100hPa, more preferably from 250 to 1,050 hPa.

On the other hand, the polymerization reaction can be suppressed by anoperation of adding an end group adjusting agent such as acid anhydridecompound, and the reaction rate and melt viscosity can be controlled byadjusting the amount added of the end group adjusting agent. Thisoperation is performed through an addition port (44). It is consideredthat the amino end group of polymer is blocked by the addition of anacid anhydride compound and thereby the polymerization reaction can besuppressed. Examples of the acid anhydride compound includehexahydrophthalic anhydride (HOPA), phthalic anhydride, trimelliticanhydride, pyromellitic anhydride and succinic anhydride. Among these,HOPA is preferred in view of color tone of the polyamide. The amountadded of the acid anhydride compound varies depending on the desiredmelt viscosity and is not particularly limited, but usually, the amountadded is preferably 150 meq/kg or less. If the amount added exceeds 150meq/kg, the polymerization rate may be decreased or vent-up may becaused and the operation stability is worsened. Furthermore, theunreacted acid anhydride compound remains in the polymer and gives riseto reduction in the quality of polyamide.

The melt viscosity of the polymer is preferably controlled by performingonly the inert gas purging operation alone out of the inert gas purgingoperation, the vacuum operation and the addition operation of an endgroup adjusting agent, or performing two or more operations out of thesethree operations.

If the vacuum operation or addition operation of an end group adjustingagent is performed alone, the following bad effects are liable to occurand this is not preferred. For example, if the fluctuation of vacuumdegree becomes excessively large, the residence amount of polymer in thepolymerization vessel is varied due to change in the holdup broughtabout by bubbling and generation of contamination, change in theresidence time or the like is caused, as a result, a polyamide having anintended quality can be hardly obtained. Also, if the fluctuation in theamount added of the end group adjusting agent becomes too large, theobtained polyamide disadvantageously has a dispersion in the end groupconcentration.

Therefore, the melt viscosity of the polymer can be stably controlledwhile preventing occurrence of these bad effects, by using a method ofperforming only the inactive gas purging operation alone andautomatically controlling the purged amount or by using two or threeoperations in combination out of the inert gas purging operation, thevacuum operation and the addition operation of an end group adjustingagent, and automatically controlling at least one operation amount inthe operations performed.

In the present invention, when two operations selected from thosedescribed above are performed, it is preferred that one operation amountout of two operation amounts is set as a fixed value and the other oneoperation amount is automatically controlled.

In the present invention, when those three operations all are performed,it is preferred that two operation amounts out of three operationamounts are each set as a fixed value and only the remaining oneoperation amount is automatically controlled or that only one operationamount out of three operation amounts is set as a fixed value and theother two operation amounts are automatically controlled.

Among these methods, the method to be employed may be appropriatelyselected by taking account of the kind of polyamide intended to produceand the objective polymerization degree.

A preferred method is to perform the vacuum operation without performingthe addition operation of an end group adjusting agent and automaticallycontrol the inert gas purged amount as a variation while settingconstant the vacuum degree. This method is advantageous in that theproduction of a polyamide with high polymerization degree unachievableby performing only the inert gas purging alone or vacuum degree alone isfacilitated and the vacuum degree can be kept at a constant value not inan excessively high region by purging with an inert gas, therebyeliminating a risk of bringing about vent-up due to excessively highvacuum degree.

In this case, the conditions are preferably such that the vacuum degreeis a constant value in the range from 400 to 1,150 hPa and the inert gaspurged amount is automatically controlled in the range from 0.005 to 9.5L/kg, more preferably such that the vacuum degree is a constant value inthe range from 450 to 1,100 hPa and the inert gas purged amount isautomatically controlled in the range from 0.01 to 9 L/kg. If the inertgas purged amount is less than 0.005 L/kg or the vacuum degree exceeds1,150 hPa, the polymerization rate decreases, whereas if the inert gaspurged amount exceeds 9.5 L/kg or the vacuum degree is less than 400hPa, this is disadvantageous in that, for example, the amount of theinert gas used increases to bring about cost rise or the polymerizationrate decreases to cause reduction in the productivity.

Another preferred method is to perform all of the addition operation ofan end group adjusting agent, the vacuum operation and the inert gaspurging operation and automatically control the inert gas purged amountas a variation while setting constant the amount added of an end groupadjusting agent and the vacuum degree. By previously determining theamount added of an end group adjusting agent, the end groupconcentration of the produced polyamide can be adjusted to a desireddegree.

In this case, the conditions are preferably such that the vacuum degreeis a constant value in the range from 400 to 1,150 hPa, the amount ofend group adjusting agent is a constant value in the range from 5 to 150meq/kg, and the inert gas purged amount is automatically controlled inthe range from 0.005 to 9.5 L/kg, more preferably such that the vacuumdegree is a constant value in the range from 450 to 1,100 hPa, theamount of end group adjusting agent is a constant value in the rangefrom 10 to 140 meq/kg, and the inert gas purged amount is automaticallycontrolled in the range from 0.01 to 9 L/kg.

FIG. 2 is a conceptual view of the system for automatically controllingthe operation amount. The melt viscosity of polymer continuouslydischarged from the outlet (45) of the final polymerization reactor iscontinuously measured by a viscometer (50). The measured viscosity valueis sent to the viscosity control unit XCA. In the viscosity control unitXCA, the viscosity value is previously set in a definite range and also,for example, which operation amount is used, which operation amount outof operation amounts used is set as a fixed value, and which operationamount is automatically controlled as a variation are set. The viscositycontrol unit XCA feeds a control signal to a system to be automaticallycontrolled out of the inert gas purging system, the vacuum system andthe addition system of end group adjusting agent, and automaticallycontrol the system. When the melt viscosity of the polymer is estimatedto come short of the set definite range, the system is controlled toincrease the inert gas purged amount, elevate the vacuum degree ordecrease the amount added of an end group adjusting agent, so as toaccelerate the polymerization reaction. On the other hand, when the meltviscosity is estimated to exceed the set definite range, the system iscontrolled to decrease the inert gas purged amount, decrease the vacuumdegree or increase the amount added of an end group adjusting agent, soas to suppress the polymerization reaction. In this way, a polymerhaving a melt viscosity in a definite range can be continuouslyproduced.

The viscometer (50) may be a conventionally known viscometer such ascapillary viscometer and vibratory viscometer, but in view offacilitated maintenance and control in the continuous production, avibratory viscometer is preferred.

As an index for knowing how exactly the automatic control of meltviscosity is performed, a standard deviation [σ] on the relativeviscosity [RV] of the polyamide sampled in aging can be used. In orderto assure the stabilized quality of polyamide, [σ] is preferably 0.12 orless, more preferably 0.09 or less.

For example, in the case of a polyamide mainly comprising ADA-MXD, the[RV] is preferably controlled by the combination of inert gas purgingand vacuum degree. This method is advantageous in that the production ofa polyamide with high [RV] unachievable by performing only the inert gasor vacuum degree alone is facilitated and [RV] can be controlled in aconstant vacuum degree by adjusting the inert gas purged amount or in aconstant inert gas amount by adjusting the vacuum degree, thus assuringmore flexible control of [RV].

The conditions are preferably such that the inert gas purged amount isfrom 0.005 to 9.5 L/kg and the vacuum degree is from 200 to 1,150 hPa,more preferably such that the inert gas purged amount is from 0.01 to 9L/kg and the vacuum degree is from 250 to 1,100 hPa. If the inert gaspurged amount is less than 0.005 L/kg or the vacuum degree exceeds 1,150hPa, the polymerization rate decreases, whereas if the inert gas purgedamount exceeds 9.5 L/kg or the vacuum degree is less than 200 hPa, thisis disadvantageous in that, for example, the amount of the inert gasused increases to bring about cost rise or the polymerization ratedecreases to cause reduction in the productivity.

The control method for [RV] is not limited to those described above, andaddition of an alkali metal compound or conventionally known varioustechniques may be employed.

EXAMPLES

The present invention is described in greater detail below by referringto Examples, but the present invention is not limited to these Examples.

[Measurement of Parameters]

1. The shear rate (γ) and shear stress (τ) were determined according tothe following formula:γ(1/sec)=8U/Dwherein U is a flow rate (cm/sec) and D is an inner diameter (cm) oftube;τ(Pa)=μ·γwherein μ is a melt viscosity (Pa·sec) and this was on-line measured bya vibratory viscometer of MANSCO disposed at the inlet of the amidationstep.2. Relative Viscosity [RV]

A polyamide (0.25 g) was dissolved in 25 ml of 96% sulfuric acid and therelative viscosity was determined from the ratio in the falling ratebetween the 96% sulfuric acid and the polyamide resin solution measuredwith use of an Ostwald viscosity tube in a constant temperature bathkept at 20° C.

3. Color Tone [Co-b]

Polyamide resin chips (10 g) were uniformly filled in a cell and thecolor tone was measured by Color Meter Model 1001DP manufactured byNippon Denshoku Industries Co., Ltd.

4. Composition of Polyamide

A polyamide resin was dissolved in hexafluoroiso-propanol and thecomposition was determined with use of Unity-500 NMR Spectroscopemanufactured by Varian.

5. Water Absorption Percentage of Polyamide

A 10 cm-square polyamide shaped plate with a thickness of 2 mm wasvacuum-dried at 100° C. for 24 hours, immediately allowed to cool in adesiccator containing silica gel and then taken out, and the dry weight(W₁) was measured. Thereafter, the shaped plate was dipped in distilledwater at 80° C. for 24 hours and after completely wiping off the wateradhering to the surface, the weight (W₂) after water absorptiontreatment was measured. The water absorption percentage (%) wasdetermined according to the following formula:Water absorption percentage (%)=[(W ₂)−(W ₁))/(W ₁) ]×1006. Amino End Group Concentration [AEG]

A polyamide resin sample (0.6 g) was dissolved in 50 ml ofphenol/ethanol (volume ratio: 4/1). Subsequently, 20 ml of water/ethanol(volume ratio: 3/2) was added was added thereto and one droplet ofindicator methyl orange was added. The resulting solution was titratedwith an aqueous ethanolic hydrochloric acid solution (prepared by addingdistilled water to 100 ml of 1/10N HCl and 50 ml of ethanol to made 500ml) and the amino end group concentration [AEG] was calculated accordingto the following formula:AEG (meq/kg)={[(A−B)×N×f]/(w×1000)}×10⁶

-   A: titration value (ml)-   B: blank titration value of solvent (ml)-   N: concentration (mol/liter) of ethanolic HCl-   f: factor of ethanolic HCl-   w: weight (g) of sample    7. Carboxyl End Group Concentration [CEG]

Benzyl alcohol (10 ml) was added to 0.2 g of a polyamide resin sampleand the sample was dissolved at 205±5° C. for 5 minutes. The resultingsolution was cooled in water for 15 seconds and after adding phenolphthalein as an indicator, titrated with an ethanolic potassiumhydroxide solution (prepared by adding ethanol to 80 ml of 0.5N-KOH tomake 1,000 ml), and the carboxyl end group concentration [CEG] wascalculated according to the following formula:CEG (meq/kg)={[(A−B)×N×f]/(w×1000)}×10⁶

-   A: titration value (ml)-   B: blank titration value of solvent (ml)-   N: concentration (mol/liter) of ethanolic potassium hydroxide-   f: factor of ethanolic potassium hydroxide-   w: weight (g) of sample    [Automatic Control of Melt Viscosity]

The melt viscosity was on-line measured by a vibratory viscometer (50)(TOV2079) of MANSCO disposed at the outlet of the final polymerizationvessel. The operations were automatically controlled through a viscositycontrol unit XCA by using an automatic on-off valve disposed in the leakpart of vacuum line in the case of vacuum degree, or by the frequency ofthe supply pump (HYM-1-010-51, manufactured by Fuji Techno IndustriesCorp.) in the case of end group adjusting agent.

Example 1

A polyamide prepolymer ([RV]:1.61) mainly comprising adipic acid andm-xylylenediamine was supplied to reactor SCR conditioned to a reactiontemperature of 255° C., an atmospheric pressure (no vacuum operation)and a screw rotation number of 50 rpm, thereby performing the finalpolymerization. Based on the indicated value of the melt viscometer, thepurged amount with nitrogen gas (purity: 99.999% or more) wasautomatically controlled. with an average residence time of 10 minutesin SCR, a polyamide resin having an average [RV] value of 2.37 and astandard deviation [σ] of 0.02 was obtained. The average [RV] value andstandard deviation [σ] were calculated from the results obtained bymeasuring 20 samples extracted at intervals of 10 minutes or more.

Example 2

The same procedure as in Example 1 was preformed except for setting thevacuum degree in the reactor SCR to a fixed value of 865 hPa. Theobtained polyamide resin had an average [RV] value of 2.38 and astandard deviation [σ] of 0.01.

Example 3

A polyamide prepolymer ([RV]:1.65) mainly comprising adipic acid,m-xylylenediamine and terephthalic acid was supplied to reactor SCRconditioned to a temperature of 255° C., a vacuum degree of 860 hPa(fixed value), a nitrogen gas (purity: 99.999% or more) purged amount of0.38 L/kg (fixed value) and a screw rotation number of 50 rpm, therebyperforming the final polymerization. With passing of about 1.5 minutesafter the addition of the prepolymer to SCR, hexahydrophthalic anhydride(HOPA) was added. The amount added of HOPA was automatically controlledbased on the indicated value of the melt viscometer. With an averageresidence time of 10 minutes, a polyamide resin having an average [RV]value of 2.41 and a standard deviation [σ] of 0.02 was obtained.

Comparative Example 1

The same procedure as in Example 1 was preformed except for performingno nitrogen gas purging in the reactor SCR and controlling the meltviscosity by automatic control using the vacuum degree. The obtainedpolyamide had an average [RV] value of 2.37, but the standard deviation[σ] was as large as 0.32 and the contamination was also large.

Comparative Example 2

The same procedure as in Example 3 was preformed except for setting thevacuum degree in the reactor SCR to atmospheric pressure (1,013 hPa) andperforming no nitrogen gas purging. The obtained polyamide had anaverage [RV] value of 2.00, but the standard deviation [σ] was 0.14revealing large dispersion. The dispersion of the end groupconcentration was also large and the quality was not satisfied.

Comparative Example 3

The same procedure as in Example 1 was preformed except for using nomelt viscometer and setting the nitrogen gas purged amount in thereactor SCR to a fixed value (3.38 L/kg). The obtained polyamide resinhad an average [RV] value of 2.10, but the standard deviation [σ] was0.32, revealing large dispersion.

Comparative Example 4

The same procedure as in Example 1 was preformed except for using ahorizontal twin-screw reaction apparatus with no self-cleaning propertyin place of the reactor SCR. The obtained polyamide resin had an average[RV] value of 2.00, but the standard deviation [σ] was 0.35, revealinglarge dispersion. Also, the contamination was large as compared withExample 1 and the quality was not satisfied.

Example 4

By referring to FIG. 1, 25 kg of powdery adipic acid (ADA) and 18 kg ofm-xylylenediamine (MXD) were supplied to the melting tank (11) and themelting tank (13), respectively. In each of the melting tanks (11) and(13), an operation of keeping a vacuum degree of 40 hPa for 5 minutesand then creating an atmospheric pressure with nitrogen gas was repeatedthree times. Thereafter, ADA and MXD were heated at 180° C. and 60° C.,respectively, under a nitrogen pressure of 0.2 MPa to obtain respectivemelted liquids. Subsequently, ADA and MXD were transferred to thestorage tank (12) and the storage tank (14), respectively.

The melted raw materials ADA and MXD were supplied by plunger pumps (15)and (16) (both, Model HYSA-JS-10, manufactured by Fuji Techno IndustriesCorp.), respectively, to the tubular reaction apparatus (L/D=780) (21)for the amidation step at a ratio of giving an equimolar ratio. At thistime, as for the mass flow rates of ADA and MXD, the outputs of theplunger pumps (15) and (16) were automatically controlled by the controlunit (19) so that the mass flowmeters (17) and (18) (both, ModelCN003D-SS-200R, Coriolis-type flowmeter, manufactured by OvalCorporation) disposed immediately after the plunger pumps (15) and (16)could show indicated values of 4.75 kg/hr and 4.42 kg/hr, respectively.

The average residence time in the amidation step was 35 minutes. Thereaction conditions in the amidation step were such that the innertemperature at the inlet (22) was 180° C., the inner temperature at theoutlet (23) was 255° C., and the inner pressure was 0.7 MPa. The shearrate γ at the inlet (22) of the amidation step was 3.1 (1/sec) and theshear stress τ was 9.3×10⁻⁴ Pa. The difference ARV between the relativeviscosity [RV] at the inlet (22) of the amidation step and the [RV] atthe outlet (23) was 0.22.

The reaction mixture passed through the amidation step was supplied tothe vertical stirring tank (31) for the initial polymerization stepconditioned to an inner temperature of 255° C., an inner pressure of 0.7MPa and a stirring at 30 rpm. The reaction mixture was allowed to stayfor 50 minutes under the same conditions and simultaneously, thecondensed water was removed by distillation. The reaction product passedthrough the initial polymerization step was supplied to SCR (41)conditioned to a reaction temperature of 255° C., a screw rotationnumber of 50 rpm and an atmospheric pressure (no vacuum operation), andthe nitrogen gas (purity: 99.999% or more) purged amount wasautomatically controlled by the indicated value of the melt viscometer(50). With an average residence time of 10 minutes in SCR, a polyamideresin having an average [RV] value of 2.06 and a standard deviation [σ]of 0.02 was obtained.

The results in Examples 1 to 4 and Comparative Examples 1 to 4 are shownin Table 1.

TABLE 1 Production Conditions Polymer Properties Final PolymerizationVessel [RV] Composition Prepolymer N₂ Purged Vacuum Amount AverageStandard of [RV] Amount Degree Added of Value Deviation Polyamide (−)(L/kg) (hPa) HOPA (meq/kg) (−) (−) Remarks Example 1 SM 1.61 automaticatmospheric none 2.37 0.02 control pressure Example 2 SM 1.61 automatic865 none 3.38 0.01 control Example 3 SMT 1.65 0.38 865 automatic 2.410.02 control Comparative SM 1.61 — automatic none 2.38 0.32 largecontamination Example 1 control Comparative SMT 1.65 — atmosphericautomatic 2.00 0.14 large dispersion of end Example 2 pressure controlgroup concentration Comparative SM 1.61 3.38 atmospheric none 2.10 0.32Example 3 pressure Comparative SM 1.61 automatic atmospheric none 2.000.35 large contamination Example 4 control pressure Example 4 SM —automatic atmospheric none 2.06 0.02 continuously performed controlpressure from raw material preparation SM: Adipicacid//m-Xylylenediamine SMT: Adipic acid/Terephthalicacid//m-Xylylenediamine

Example 5

Powdery adipic acid (ADA) (25 kg) and 18 kg of m-xylylenediamine (MXD)were supplied to the melting tank (11) and the melting tank (13),respectively. In each of the melting tanks (11) and (13), an operationof keeping a vacuum degree of 40 hPa for 5 minutes and then creating anatmospheric pressure with nitrogen gas was repeated three times.Thereafter, ADA and MXD were heated at 180° C. and 60° C., respectively,under a nitrogen pressure of 0.2 MPa to obtain respective meltedliquids. Subsequently, ADA and MXD were transferred to the storage tank(12) and the storage tank (14), respectively.

The melted raw materials ADA and MXD were supplied by plunger pumps (15)and (16), respectively, to the tubular reaction apparatus (L/D=780) (21)for the amidation step at a ratio of giving an equimolar ratio. At thistime, the supply amount was adjusted so that the average residence timein the amidation step could be 35 minutes. The reaction conditions inthe amidation step were such that the inner temperature at the inlet(22) was 180° C., the inner temperature at the outlet (23) was 255° C.,and the inner pressure was 0.7 MPa. The shear rate γ at the inlet (22)of the amidation step was 3.1 (1/sec) and the shear stress τ was9.3×10⁻⁴ Pa. The difference ARV between the relative viscosity [RV] atthe inlet (22) of the amidation step and the [RV] at the outlet (23) was0.22.

The reaction mixture passed through the amidation step was supplied tothe vertical stirring tank (31) for the initial polymerization stepconditioned to an inner temperature of 255° C., an inner pressure of 0.7MPa and a stirring at 30 rpm. The reaction mixture was allowed to stayfor 50 minutes under the same conditions and simultaneously, thecondensed water was removed by distillation. The reaction product passedthrough the initial polymerization step was supplied to SCR (41)conditioned to a reaction temperature of 255° C., a vacuum degree of1,013 hPa, a nitrogen gas purged amount of 1.13 L/kg and a screwrotation number of 50 rpm. With an average residence time of 10 minutes,a polyamide resin having [RV] of 2.06 and [Co-b] of 0.2 was obtained.

Examples 6 and 7

The same procedure as in Example 5 was performed except for changing theL/D by lengthening the extension distance of the tubular reactionapparatus (21) for the amidation step, and changing the amount of rawmaterial supplied to the reaction apparatus (21). The results are shownin Tables 2 and 3.

Example 8 Salt Forming Method

In the raw material salt forming tank, 10.267 kg of terephthalic acid(TPA), 11.104 kg of an aqueous 64.7 wt % hexamethylenediamine (HMDA)solution, 14.052 kg of caprolactam (CLM) and 9.577 kg of water wereadded. Then, after keeping a vacuum and creating an atmospheric pressurewith nitrogen gas similarly to Example 5, nitrogen purging at a nitrogenpressure of 0.22 MPa was performed three times. Thereafter, a 70 wt %solution of aminocarboxylate was prepared with stirring at 135° C. undera nitrogen pressure of 0.22 MPa, and the prepared solution wastransferred to the storage tank.

The aqueous aminocarboxylate solution was supplied by the plunger pumpto the tubular reaction apparatus (L/D=780) for the amidation step. Thetubular reaction apparatus was conditioned such that the innertemperature was from 135° C. (inlet) to 255° C. (outlet) and the innerpressure was from 0.22 to 2 MPa. The average residence time was about 20minutes. The shear rate γ at the inlet of the amidation step was 5.35(1/sec) and the shear stress τ was 10.7×10⁻⁴ Pa. The difference ARVbetween the relative viscosity [RV] at the inlet of the amidation stepand the [RV] at the outlet was 0.25.

The reaction product passed through the amidation step was transferredto the vertical stirring tank for the initial polymerization stepconditioned to 2 MPa and 285° C., and water produced by the reactionwith stirring at 30 rpm for 30 minutes and water used for the adjustmentof salt concentration were removed by distillation to obtain an initialpolymerization product.

The obtained initial polymerization product was supplied to SCRconditioned to a temperature of 285° C., a vacuum degree of 860 hPa anda screw rotation number of 70 rpm, thereby performing the finalpolymerization. With passing of about 1.5 minutes after the supply ofthe initial polymerization product to SCR, hexahydrophthalic anhydride(HOPA) was added. The frequency of the plunger pump was set so that theamount added of HOPA could be 45 meq/kg per 1 kg of the polymer. With anaverage residence time of about 9 minutes in SCR, a polyamide having[RV] of 2.14 and [Co-b] of 2.1 was obtained.

Example 9 Salt Forming Method

The same procedure as in Example 8 was performed except for changing theL/D by lengthening the extension distance of the tubular reactionapparatus (21) for the amidation step.

The production conditions for polyamides in Examples 5 and 9 are shownin Table 2 and the properties of each polyamide obtained are shown inTable 3. In all Examples, the operation was good and a polyamide havingexcellent physical property values was obtained.

TABLE 2 Example 5 Example 6 Example 7 Example 8 Example 9 Composition ofpolyamide MA MA MA HCT HCT Raw Oxygen removing 40 hPa × 3 + 40 hPa × 3 +40 hPa × 3 + 40 hPa × 3 + 40 hPa × 3 + material method (vacuum 0.2 MPa0.2 MPa 0.2 MPa 0.22 MPa 0.22 MPa preparation degree × times, stepnitrogen pressure) Oxygen concen- 4 4 4 4 4 tration (ppm) Amidation L/D780 1320 1320 780 1320 step Average residence 35 57 38 20 35 time (min)γ (1/sec) 3.10 3.10 4.65 5.35 5.35 τ (×10⁻⁴ Pa) 9.3 9.3 14.0 10.7 10.7ΔRV 0.22 0.24 0.24 0.25 0.25 Later N₂ Purged amount 1.13 1.13 1.13 0 0polymeri- (L/kg) zation step Vacuum degree 1013 1013 1013 860 860 (hPa)Amount added of 0 0 0 45 45 HOPA (meq/kg) MA: ADA//MXD = 100//100 (mol%) HCT: TPA/CLM//HMDA = 35/65//100 (mol %)

TABLE 3 Exam- Exam- Exam- Exam- Exam- ple 5 ple 6 ple 7 ple 8 ple 9[RV](−) 2.06 2.02 1.90 2.14 2.15 [Co-b](−) 0.2 1.0 0.4 2.1 2.1 Waterabsorption 5.4 5.4 5.2 6.3 6.3 percentage (%)

Example 10

Powdery adipic acid (ADA) (25 kg) and 18 kg of m-xylylenediamine (MXD)were supplied to the melting tank (11) and the melting tank (13),respectively. In each of the melting tanks (11) and (13), an operationof keeping a vacuum degree of 40 hPa for 5 minutes and then creating anatmospheric pressure with nitrogen gas was repeated three times.Thereafter, ADA and MXD were heated at 180° C. and 60° C., respectively,under a nitrogen pressure of 0.2 MPa to obtain respective meltedliquids. Subsequently, ADA and MXD were transferred to the storage tank(12) and the storage tank (14), respectively.

The melted raw materials ADA and MXD were supplied by plunger pumps (15)and (16) (both, Model HYSA-JS-10, manufactured by Fuji Techno IndustriesCorp.), respectively, to the tubular reaction apparatus (L/D=780) (21)for the amidation step each in a constant amount. As for the mass flowrates of ADA and MXD, the outputs of the plunger pumps (15) and (16)were automatically controlled by the control unit (19) so that the massflowmeters (17) and (18) (both, Model CN003D-SS-200R, Coriolis-typeflowmeter, manufactured by Oval Corporation) disposed immediately afterthe plunger pumps (15) and (16) could show indicated values of 4.75kg/hr and 4.42 kg/hr, respectively. The reaction conditions in theamidation step were such that the inner temperature at the inlet (22)was 180° C., the inner temperature at the outlet (23) was 255° C., theinner pressure was 0.7 MPa, and the average residence time was 30minutes.

The reaction mixture passed through the amidation step was supplied tothe vertical stirring tank (31) for the initial polymerization stepconditioned to an inner temperature of 255° C., an inner pressure of 0.7MPa and a stirring at 30 rpm. The reaction mixture was allowed to stayfor 50 minutes under the same conditions and simultaneously, thecondensed water was removed by distillation. The reaction product passedthrough the initial polymerization step was supplied to SCR (41)conditioned to a reaction temperature of 255° C., a vacuum degree of1,013 hPa, a nitrogen gas purged amount of 1.13 L/kg and a screwrotation number of 50 rpm. After passing of an average residence time of10 minutes, the polyamide was continuously ejected. The polyamide wassampled 5 times at intervals of 30 minutes and the polyamides sampledwere measured for [RV], [AEG] and [CEG]. The average value, standarddeviation and range (difference between maximum value and minimum valuein polyamides sampled 5 times) of each of [RV], [AEG] and [CEG] areshown in Table 4.

Example 11

The same procedure as in Example 10 was performed except for changingthe set mass flow rates of ADA and MXD to 4.63 kg/hr and 4.27 kg/hr,respectively.

Example 12

Powdery adipic acid (ADA) (25 kg) and 18 kg of liquidhexamethylenediamine (HMD) were supplied to the melting tank (11) andthe melting tank (13), respectively. In each of the melting tanks (11)and (13), an operation of keeping a vacuum degree of 40 hPa for 5minutes and then creating an atmospheric pressure with nitrogen gas wasrepeated three times. Thereafter, ADA and HMD were heated at 180° C. and60° C., respectively, under a nitrogen pressure of 0.2 MPa to obtainrespective melted liquids. Subsequently, ADA and HMD were transferred tothe storage tank (12) and the storage tank (14), respectively.

The melted raw materials ADA and HMD were supplied by plunger pumps (15)and (16) (the same as those used in Example 1), respectively, to thetubular reaction apparatus (L/D=780) (21) for the amidation step each ina constant amount. As for the mass flow rates of ADA and HMD, theoutputs of the plunger pumps (15) and (16) were automatically controlledby the control unit (19) so that the mass flowmeters (17) and (18) (thesame as those used in Example 10) disposed immediately after the plungerpumps (15) and (16) could show indicated values of 4.25 kg/hr and 3.38kg/hr, respectively. The reaction conditions in the amidation step weresuch that the inner temperature at the inlet (22) was 180° C., the innertemperature at the outlet (23) was 270° C., the inner pressure was 1.0MPa, and the average residence time was 30 minutes.

The reaction mixture passed through the amidation step was supplied tothe vertical stirring tank (31) for the initial polymerization stepconditioned to an inner temperature of 270° C., an inner pressure of 1.0MPa and a stirring at 30 rpm. The reaction mixture was allowed to stayfor 50 minutes under the same conditions and simultaneously, thecondensed water was removed by distillation. The reaction product passedthrough the initial polymerization step was supplied to SCR (41)conditioned to a reaction temperature of 270° C., a vacuum degree of1,013 hPa, a nitrogen gas purged amount of 0.3 L/kg and a screw rotationnumber of 50 rpm. After passing of an average residence time of 10minutes, the polyamide was continuously ejected. The polyamide wassampled 5 times at intervals of 30 minutes and the polyamides sampledwere measured for [RV], [AEG] and [CEG].

Example 13

The same procedure as in Example 10 was performed except for onceadjusting the outputs of the plunger pumps (15) and (16) so that themass flowmeters (17) and (18) of ADA and MXD could show indicated valuesof 4.75 kg/hr and 4.42 kg/hr, respectively, and keeping constant theas-adjusted outputs of the pumps (15) and (16) without automaticallycontrolling the outputs of the pumps (15) and (16) by using theindicated values of the mass flowmeters (17) and (18) and using thecontrol unit (19).

Example 14

The same procedure as in Example 10 was performed except for changingthe flowmeter from the mass flowmeter to the volume flowmeter andautomatically controlling the outputs of the plunger pumps (15) and (16)by the control unit (19) so that the volume flowmeters of ADA and MXDcould show indicated values of 5.23 L/hr and 4.62 L/hr, respectively.

TABLE 4 Example 10 Example 11 Example 12 Example 13 Example 14 RawMaterials ADA/MXD ADA/MXD ADA/HMD ADA/MXD ADA/MXD Production Conditions(set Dicarboxylic acid 4.75 kg/hr 4.63 kg/hr 4.25 kg/hr — 5.23 L/hrvalue of flow rate) Diamine 4.42 kg/hr 4.27 kg/hr 3.38 kg/hr — 4.62 L/hrPolymer [RV] Average value 2.26 2.23 1.91 1.84 2.13 Properties (−)Standard deviation 0.04 0.03 0.04 0.13 0.08 Range 0.15 0.08 0.14 0.360.34 [AEG] Average value 73.1 45.9 62.5 56.7 72.8 (meq/kg) Standarddeviation 6.2 6.7 6.8 32.5 10.9 Range 17.7 20.4 18.2 98.3 41.5 [CEG]Average value 67.2 90.7 72.3 155.9 72.9 (meq/kg) Standard deviation 7.05.4 7.9 65.5 19.5 Range 22.5 12.9 24.8 207.6 76.4

Production conditions of polyamides in Examples 10 to 14 and theproperties of each polyamide obtained are shown in Table 4. In Examples10 to 12, the operation was good and a polyamide having very homogeneousphysical properties was obtained. As seen from Examples 10 and 11, apolyamide having objective properties could be very homogeneouslyproduced depending on setting of the supply mass flow rates of ADA andMXD.

On the other hand, in Example 13, despite the same mass flow rates insupplying raw materials, the obtained polyamide underwent largefluctuation of physical properties in aging. This is considered toresult because when the flow rates in supplying raw materials are setonly by the outputs of plunger pumps, ADA and MXD cannot be suppliedeach at a constant mass flow rate at all times and the mol balancebetween ADA and MXD is fluctuated.

In Example 14, the outputs of plunger pumps were automaticallycontrolled by the control unit, nevertheless, the obtained polyamideunderwent large fluctuation of physical properties as compared withExamples 10 and 11. That is, the control by the volume flowmeter was notso exact as that by the mass flowmeter.

It is revealed from the results in Examples 10 to 14 that two melted rawmaterials are preferably supplied by using a mass flow meter.

Example 15 Salt Forming Method

In the raw material salt forming tank, 1.111 kg of terephthalic acid(TPA), 7.771 kg of m-xylylenediamine (MXD), 8.797 kg of adipic acid(ADA) and 17.679 kg of water were added. Then, nitrogen purging at anitrogen pressure of 0.2 MPa was performed three times. Thereafter, a 50wt % solution of aminocarboxylate was prepared with stirring at 135° C.under a nitrogen pressure of 0.2 MPa, and the prepared solution wastransferred to the storage tank.

The aqueous aminocarboxylate solution was supplied by the plunger pumpto the tubular reaction apparatus (L/D=780) for the amidation step. Thetubular reaction apparatus was conditioned such that the innertemperature was from 135° C. (inlet) to 265° C. (outlet) and the innerpressure was from 0.22 to 2.5 MPa. The average residence time was about35 minutes. The shear rate γ at the inlet of the amidation step was 3.10(1/sec) and the shear stress τ was 6.2×10⁻⁴ Pa. The difference ARVbetween the relative viscosity [RV] at the inlet of the amidation stepand the [RV] at the outlet was 0.25.

The reaction product passed through the amidation step was transferredto the vertical stirring tank for the initial polymerization stepconditioned to 2.5 MPa and 265° C., and water produced by the reactionwith stirring at 30 rpm for 30 minutes and water used for the adjustmentof salt concentration were removed by distillation to obtain an initialpolymerization product.

The obtained initial polymerization product was supplied to SCRconditioned to a temperature of 265° C. and a screw rotation number of50 rpm, thereby performing the final polymerization. The vacuum degreewas automatically controlled by the indexed value of the meltviscometer. With an average residence time of 10 minutes in SCR, apolyamide resin in which the average [RV] value was 2.10 and thestandard deviation [σ] of [RV] was 0.02 was obtained.

Example 16

Powdery adipic acid (ADA) (25 kg) and 20 kg of flakedhexamethylenediamine (HMDA) were supplied to the melting tank (11) andthe melting tank (13), respectively. In each of the melting tanks (11)and (13), an operation of keeping a vacuum degree of 40 hPa for 5minutes and then creating an atmospheric pressure with nitrogen gas wasrepeated three times. Thereafter, ADA and HMDA were heated at 180° C.and 60° C., respectively, under a nitrogen pressure of 0.2 MPa to obtainrespective melted liquids. Subsequently, ADA and HMDA were transferredto the storage tank (12) and the storage tank (14), respectively.

The melted raw materials ADA and HMDA were supplied by plunger pumps(15) and (16), respectively, to the tubular reaction apparatus (L/D=780)(21) for the amidation step each in a constant amount. At this time, asfor the mass flow rates of ADA and HMDA, the outputs of the plungerpumps (15) and (16) were automatically controlled by the control unit(19) so that the mass flowmeters (17) and (18) (both, ModelCN003D-SS-200R, Coriolis-type flowmeter, manufactured by OvalCorporation) disposed immediately after the plunger pumps (15) and (16)could indicate the values shown in Table 5, respectively.

The average residence time in the amidation step was 35 minutes. Thereaction conditions in the amidation step were such that the innertemperature at the inlet (22) was 180° C., the inner temperature at theoutlet (23) was 270° C., and the inner pressure was 0.7 MPa. The shearrate γ at the inlet (22) of the amidation step was 3.1 (1/sec) and theshear stress τ was 9.0×10⁻⁴ Pa. The difference ARV between the relativeviscosity [RV] at the inlet (22) of the amidation step and the [RV] atthe outlet (23) was 0.23.

The reaction mixture passed through the amidation step was supplied tothe vertical stirring tank (31) for the initial polymerization stepconditioned to an inner temperature of 270° C., an inner pressure of 0.7MPa and a stirring at 30 rpm. The reaction mixture was allowed to stayfor 50 minutes under the same conditions and simultaneously, thecondensed water was removed by distillation. The reaction product passedthrough the initial polymerization step was supplied to reactor SCRconditioned to a reaction temperature of 270° C., 666 hPa (fixed value)and a screw rotation number of 50 rpm, thereby performing the finalpolymerization. The nitrogen gas (purity: 99.999% or more) purged amountwas automatically controlled by the indicated value of the meltviscometer (50). With an average residence time of 10 minutes in SCR, apolyamide resin having an average [RV] value of 2.95 and a standarddeviation [σ] of 0.03 was obtained.

Example 17

Powdery adipic acid (ADA) (22.5 kg) and 2.94 kg of powdery1,4-cyclohexanedicarboxylic acid (CHDA) were supplied to the meltingtank (11), and 18 kg of m-xylylenediamine (MXD) was supplied to themelting tank (13). In each of the melting tanks (11) and (13), anoperation of keeping a vacuum degree of 40 hPa for 5 minutes and thencreating an atmospheric pressure with nitrogen gas was repeated threetimes. Thereafter, the ADA/CHDA mixture and MXD were heated at 180° C.and 60° C., respectively, under a nitrogen pressure of 0.2 MPa to obtainrespective melted liquids. Subsequently, the ADA/CHDA mixed liquid andMXD were transferred to the storage tank (12) and the storage tank (14),respectively.

The melted raw materials, that is, ADA/CHDA mixture and MXD, weresupplied by plunger pumps (15) and (16), respectively, to the tubularreaction apparatus (L/D=780) (21) for the amidation step each in aconstant amount. At this time, as for the mass flow rates of ADA andCHDA, the outputs of the plunger pumps (15) and (16) were automaticallycontrolled by the control unit (19) so that the mass flowmeters (17) and(18) (both, Model CN003D-SS-200R, Coriolis-type flowmeter, manufacturedby Oval Corporation) disposed immediately after the plunger pumps (15)and (16) could indicate the values shown in Table 5, respectively.

The average residence time in the amidation step was 35 minutes. Thereaction conditions in the amidation step were such that the innertemperature at the inlet (22)was 180° C., the inner temperature at theoutlet (23) was 255° C., and the inner pressure was 0.7 MPa. The shearrate γ at the inlet (22) of the amidation step was 3.1 (1/sec) and theshear stress τ was 9.6×10⁻⁴ Pa. The difference ARV between the relativeviscosity [RV] at the inlet (22) of the amidation step and the [RV] atthe outlet (23) was 0.22.

The reaction mixture passed through the amidation step was supplied tothe vertical stirring tank (31) for the initial polymerization stepconditioned to an inner temperature of 255° C., an inner pressure of 0.7MPa and a stirring at 30 rpm. The reaction mixture was allowed to stayfor 50 minutes under the same conditions and simultaneously, thecondensed water was removed by distillation. The reaction product passedthrough the initial polymerization step was supplied to SCR (41)conditioned to a reaction temperature of 255° C., a screw rotationnumber of 50 rpm and a vacuum degree of 1,000 hPa, and the nitrogen gas(purity: 99.999% or more) purged amount was controlled by the indicatedvalue of the melt viscometer (50). With an average residence time of 10minutes in SCR, a polyamide resin having an average [RV] value of 2.23and a standard deviation [σ] of 0.03 was obtained.

The results of Examples 16 and 17 are shown in Table 5.

These Examples are mere examples in all respects and should not beconstrued as restrictive. Furthermore, all changes and modificationsbelonging to the equivalent range of the scope of claim for a patent arewithin the scope of the present invention.

TABLE 5 Example 16 Example 17 Production Composition of Polyamide 6,6SMC Conditions Raw Material Concentration of oxygen (ppm) 4 4Preparation Step Raw Material Set value of flow rate dicarboxylic acid5.10 kg/hr 4.83 kg/hr Introduction diamine 4.06 kg/hr 4.42 kg/hr StepAmidation L/D (−) 780 780 Step Average residence time (min) 35 35 γ(1/sec) 3.10 3.10 τ (×10⁻⁴ Pa) 9.0 9.6 ΔRV (−) 0.23 0.22 Later Nitrogenpurged amount (L/kg) automatic automatic Polymeri- control controlzation Step Vacuum degree (hPa) 666 1000 Amount added of HOPA (meq/kg)none none Properties [RV] Average value (−) 2.95 2.23 of PolymerStandard deviation (−) 0.03 0.03 6,6: adipic acid//hexamethylenediamineSMC: adipic acid/1,4-cyclohexanedicarboxylic acid//m-xylylenediamine

1. A continuous production method of a polyamide, comprisingcontinuously producing the polyamide by melt polymerization using amultistage polymerization reaction apparatus, wherein a self-cleaninghorizontal twin-screw reaction apparatus is used as a finalpolymerization reaction apparatus constituting the multistagepolymerization reaction apparatus, wherein the final polymerization iseffected while performing an operation of purging inert gas inside thefinal polymerization reaction apparatus or while performing two or threeoperations selected from the group consisting of an operation of purginginert gas inside the final polymerization reaction apparatus, anoperation of vacuating the final polymerization reaction apparatus, andan operation of adding an end group adjusting agent into the finalpolymerization reaction apparatus, and wherein the melt viscosity of thepolyamide is controlled by continuously measuring the melt viscosity ofthe polyamide at an outlet of the final polymerization reactionapparatus by a viscometer and automatically controlling at least oneoperation amount selected from the group consisting of the amount of theinert gas purged, the vacuum degree and the amount added of the endgroup adjusting agent corresponding to said operations so that themeasured viscosity value becomes a value within a previously setdefinite range.
 2. The continuous production method of a polyamide asclaimed in claim 1, wherein two operations selected from the groupconsisting of the inert gas purging operation, the vacuum operation andthe addition operation of an end group adjusting agent are performed andone operation amount out of two operation amounts is set as a fixedvalue and the other operation amount is automatically controlled.
 3. Thecontinuous production method of a polyamide as claimed in claim 1,wherein all the three operations selected from the group consisting ofthe inert gas purging operation, the vacuum operation and the additionoperation of an end group adjusting agent are performed and twooperation amounts out of three operation amounts are each set as a fixedvalue and only the remaining one operation amount is automaticallycontrolled, or only one operation amount out of three operation amountsis set as a fixed value and the other two operation amounts areautomatically controlled.
 4. The continuous production method of apolyamide as claimed in claim 1, wherein the inert gas has a moisturepercentage of 0.05 wt % or less.
 5. The continuous production method ofa polyamide as claimed in claim 1, wherein the polyamide comprises anm-xylylenediamine (MXD) as a diamine component, and them-xylylenediamine (MXD) content is at least 70 mol % based on thediamine component.
 6. The continuous production method of a polyamide asclaimed in claim 1, wherein a polyamide having a relative viscosity [RV]of 1.6 to 4.0 is obtained.
 7. A continuous production method of apolyamide mainly comprising a diamine component unit and a dicarboxylicacid component unit, said method comprising: (a) a raw materialpreparation step of individually melting a diamine and a dicarboxylicacid or forming a salt of amine and carboxylic acid in water, (b) a rawmaterial introduction step of continuously introducing the prepared rawmaterial into a tubular reaction apparatus, (c) an amidation step ofpassing the introduced raw material through the tubular reactionapparatus, thereby effecting amidation to obtain a reaction mixturecontaining an amidated product and condensed water, (d) an initialpolymerization step of introducing said reaction mixture into acontinuous reaction apparatus capable of separation and removal ofwater, and elevating the polymerization degree while separating andremoving water at a temperature higher than the melting point of thefinally obtained polyamide to obtain a potyamide prepolymer, and (e) afinal polymerization step of introducing the polyamide prepolymer into acontinuous reaction apparatus capable of separation and removal ofwater, and further elevating the polymerization degree at a temperaturehigher than the melting point of the finally obtained polyamide toobtain a polyamide adjusted to a desired relative viscosity [RV] whereinin the final polymerization step (e), the final polymerization iseffected while performing an operation of purging inert gas inside thereaction apparatus or while performing two or three operations selectedfrom the group consisting of an operation of purging inert gas insidethe reaction apparatus, an operation of vacuating the reactionapparatus, and an operation of adding an end group adjusting agent intothe reaction apparatus, and wherein the melt viscosity of the polyamideis controlled by continuously measuring the melt viscosity of thepolyamide at an outlet of the final polymerization reaction apparatus bya viscometer and automatically controlling at least one operation amountout of the inert gas purged amount, the vacuum degree and the amountadded of the end group adjusting agent corresponding to said operationsso that the measured viscosity value becomes a value within a previouslyset definite range.
 8. The continuous production method of a polyarnideas claimed in claim 7, wherein the tubular reaction apparatus used forthe amidation step (c) has L/D of 50 or more, wherein the inner diameterof the tube is D (mm) and the length of the tube is L (mm).
 9. Thecontinuous production method of a polyamide as claimed in claim 7,wherein the average residence time in the amidation step (c) is from 10to 120 minutes.
 10. The continuous production method of a polyamide asclaimed in claim 7, wherein the shear rate (γ) in the amidation step (c)is 0.1 (1/see) or more and the shear stress (τ) is 1.5×10⁻⁵ Pa or more.11. The continuous production method of a polyamide as claimed in claim7, wherein in the amidation step (c), the relative viscosity [RV] of thereaction mixture is elevated by 0.05 to 0.6.
 12. The continuousproduction method of a polyamide as claimed in claim 7, wherein theaverage residence time in the initial polymerization step (d) is from 10to 150 minutes.
 13. The continuous production method of a polyamide asclaimed in claim 7, wherein the continuous reaction apparatus in thefinal polymerization step (e) is a horizontal reaction apparatus. 14.The continuous production method of a polyamide as claimed in claim 7,wherein the continuous reaction apparatus in the final polymerizationstep (e) is a self-cleaning horizontal twin-screw reaction apparatus.15. The continuous production method of a polyamide as claimed in claim7, wherein the average residence time in the final polymerization step(e) is from 1 to 30 minutes.
 16. The continuous production method of apolyamide as claimed in claim 7, wherein the relative viscosity [RV] ofthe polyamide obtained in the final polymerization step (e) is from 1.6to 4.0.
 17. The continuous production method of a polyamide as claimedin claim 7, wherein in the final polymerization step (e), the relativeviscosity [RV] of the polyamide is controlled by an operation of purginginert gas inside the reaction apparatus, an operation of adjustingvacuum degree in the reaction apparatus, an operation of adding an endgroup adjusting agent into the reaction apparatus, or a combinationthereof.
 18. The continuous production method of a polyamide as claimedin claim 7, wherein in the raw material preparation step (a), theatmospheric oxygen concentration at the preparation of raw material is10 ppm or less.
 19. The continuous production method of a polyamide asclaimed in claim 7, wherein the polyamide comprises at least one memberselected from the group consisting of the following repeating units (I)to (IV) and optionally further comprises the following repeating unit(V):


20. The continuous production method of a polyamide as claimed in claim19, wherein the polyamide comprises at least one member selected fromthe group consisting of the repeating units (I), (III) and (IV).
 21. Thecontinuous production method of a polyamide as claimed in claim 7,wherein the polyamide comprises an m-xylylenediamine (MXD) as a diaminecomponent, and the m-xylyienediamine (MXD) content is at least 70 mol %based on the diamine component.
 22. A continuous production method of apolyamide mainly comprising a diamine component unit and a dicarboxylicacid component unit, said method comprising: (a) a raw materialpreparation step of individually preparing a melted diamine and a melteddicarboxylic acid, (b) a raw material introduction step of continuouslyintroducing the melted diamine and the melted dicarboxylic acid into atubular reaction apparatus to get a diamine and a carboxylic acidtogether by using raw material supply means comprising a raw materialsupply device, a mass flow rate measuring device provided on thedownstream side of said raw material supply device and a control systemof automatically controlling the output of said supply device such thatthe mass flow rate measured by said mass flow rate measuring devicebecomes a previously set value, (c) an amidation step of passing thediarnine and dicarboxylic acid gotten together through the tubularreaction apparatus, thereby effecting amidation to obtain a reactionmixture containing an amidated product and condensed water, (d) aninitial polymerization step of introducing said reaction mixture into acontinuous reaction apparatus capable of separation and removal ofwater, and elevating the polymerization degree while separating andremoving water at a temperature higher than the melting point of thefinally obtained polyamide to obtain a polyamide prepolymer, and (e) afinal polymerization step of introducing the polyamide prepolymer into aself-cleaning horizontal twin-screw reaction apparatus capable ofseparation and removal of water, and further elevating thepolymerization degree at a temperature higher than the melting point ofthe finally obtained polyamide to obtain a polyamide adjusted to adesired relative viscosity [RV] wherein in the final polymerization step(e), the final polymerization is effected while performing an operationof purging inert gas inside the reaction apparatus or while performingtwo or three operations selected from the group consisting of anoperation of purging inert gas inside the reaction apparatus, anoperation of vacuating the reaction apparatus, and an operation ofadding an end group adjusting agent into the reaction apparatus, andwherein the melt viscosity of the polyamide is controlled bycontinuously measuring the melt viscosity of the polyamide at an outletof the final polymerization reaction apparatus by a viscometer andautomatically controlling at least one operation amount out of the inertgas purged amount, the vacuum degree and the amount added of the endgroup adjusting agent corresponding to said operations so that themeasured viscosity value becomes a value within a previously setdefinite range.
 23. The continuous production method of a polyamide asclaimed in claim 22, wherein the tubular reaction apparatus used for theamidation step (c) has L/D of 50 or more, wherein the inner diameter ofthe tube is D (mm) and the length of the tube is L (mm).
 24. Thecontinuous production method of a polyamide as claimed in claim 22,wherein the average residence time in the final polymerization step (e)is from 1 to 30 minutes.
 25. The continuous production method of apolyamide as claimed in claim 22, wherein the relative viscosity [RV] ofthe polyamide obtained in the final polymerization step (e) is from 1.6to 4.0.
 26. The continuous production method of a polyamide as claimedin claim 22, wherein in the final polymerization step (e), the relativeviscosity [RV] of the polyamide is controlled by an operation of purginginert gas inside the reaction apparatus, an operation of adjustingvacuum degree in the reaction apparatus, an operation of adding an endgroup adjusting agent into the reaction apparatus, or a combinationthereof.
 27. The continuous production method of a polyamide as claimedin claim 22, wherein in the raw material preparation step (a), theatmospheric oxygen concentration at the preparation of raw material is10 ppm or less.
 28. The continuous production method of a polyamide asclaimed in claim 22, wherein the polyamide comprises anm-xylylenediamine (MXD) as a diamine component, and them-xylylenediamine (MXD) content is at least 70 mol % based on thediamine component.
 29. The continuous production method of a polyamideas claimed in claim 22, wherein the relative viscosity [RV] of thepolyamide obtained in the final polymerization step (e) is from 1.6 to4.0.